diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index 7e8a1f00107a4..2caad649ce4e7 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -2151,7 +2151,6 @@ if (!is_android) {
         "//ui/file_manager:unit_test_data",
         "//ui/file_manager/file_manager/common/js:modulize_runtime_data",
         "//ui/file_manager/file_manager/externs:modulize_runtime_data",
-        "//ui/file_manager/file_manager/externs/background:modulize_runtime_data",
         "//ui/file_manager/file_manager/foreground/js:modulize_runtime_data",
       ]
 
diff --git a/ui/file_manager/BUILD.gn b/ui/file_manager/BUILD.gn
index 8cffec47601fa..2dae24b34e9d5 100644
--- a/ui/file_manager/BUILD.gn
+++ b/ui/file_manager/BUILD.gn
@@ -86,6 +86,17 @@ preprocess_if_expr("preprocess_static") {
     "audio_player/js/test_util.js",
 
     # Metadata:
+    "file_manager/externs/background/import_history.js",
+    "file_manager/externs/background/task_queue.js",
+    "file_manager/externs/background/progress_center.js",
+    "file_manager/externs/background/media_scanner.js",
+    "file_manager/externs/background/media_import_handler.js",
+    "file_manager/externs/background/file_operation_manager.js",
+    "file_manager/externs/background/duplicate_finder.js",
+    "file_manager/externs/background/drive_sync_handler.js",
+    "file_manager/externs/background/crostini.js",
+    "file_manager/externs/background/background_base.js",
+    "file_manager/externs/background/file_browser_background_full.js",
     "file_manager/foreground/js/metadata/content_metadata_provider.js",
     "file_manager/foreground/js/metadata/byte_reader.js",
     "file_manager/foreground/js/metadata/thumbnail_model.js",
@@ -282,17 +293,6 @@ preprocess_if_expr("preprocess_generated") {
     "file_manager/common/js/xfm.m.js",
 
     # Externs:
-    "file_manager/externs/background/background_base.m.js",
-    "file_manager/externs/background/crostini.m.js",
-    "file_manager/externs/background/drive_sync_handler.m.js",
-    "file_manager/externs/background/duplicate_finder.m.js",
-    "file_manager/externs/background/file_browser_background_full.m.js",
-    "file_manager/externs/background/file_operation_manager.m.js",
-    "file_manager/externs/background/import_history.m.js",
-    "file_manager/externs/background/media_import_handler.m.js",
-    "file_manager/externs/background/media_scanner.m.js",
-    "file_manager/externs/background/progress_center.m.js",
-    "file_manager/externs/background/task_queue.m.js",
     "file_manager/externs/background_window.m.js",
     "file_manager/externs/command_handler_deps.m.js",
     "file_manager/externs/directory_change_event.m.js",
@@ -382,7 +382,6 @@ preprocess_if_expr("preprocess_generated") {
     "//ui/file_manager/audio_player/elements:web_components",
     "//ui/file_manager/file_manager/common/js:modulize",
     "//ui/file_manager/file_manager/externs:modulize",
-    "//ui/file_manager/file_manager/externs/background:modulize",
     "//ui/file_manager/file_manager/foreground/elements:elements",
     "//ui/file_manager/file_manager/foreground/elements:web_components",
     "//ui/file_manager/file_manager/foreground/js:modulize",
@@ -443,7 +442,6 @@ generate_grd("build_grd") {
       "file_manager/common/js/test_error_reporting.m.js",
       "file_manager/common/js/volume_manager_types.m.js",
       "file_manager/common/js/xfm.m.js",
-      "file_manager/externs/background/background_base.m.js",
       "file_manager/externs/entry_location.m.js",
       "file_manager/externs/files_app_entry_interfaces.m.js",
       "file_manager/externs/volume_info.m.js",
diff --git a/ui/file_manager/file_manager/BUILD.gn b/ui/file_manager/file_manager/BUILD.gn
index 13f5095022f82..3d8d88c914815 100644
--- a/ui/file_manager/file_manager/BUILD.gn
+++ b/ui/file_manager/file_manager/BUILD.gn
@@ -31,6 +31,7 @@ generate_grd("build_static_grdp") {
 
     # For some unittests:
     "foreground/js/metadata/image_orientation.js",
+    "externs/background/background_base.js",
 
     # CSS:
     "foreground/css/combobutton.css",
diff --git a/ui/file_manager/file_manager/background/js/BUILD.gn b/ui/file_manager/file_manager/background/js/BUILD.gn
index 7b5cdeb493141..b5f4473b61ae3 100644
--- a/ui/file_manager/file_manager/background/js/BUILD.gn
+++ b/ui/file_manager/file_manager/background/js/BUILD.gn
@@ -153,15 +153,15 @@ js_library("background") {
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:crostini.m",
-    "//ui/file_manager/file_manager/externs/background:drive_sync_handler.m",
-    "//ui/file_manager/file_manager/externs/background:duplicate_finder.m",
-    "//ui/file_manager/file_manager/externs/background:file_browser_background_full.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
-    "//ui/file_manager/file_manager/externs/background:media_import_handler.m",
-    "//ui/file_manager/file_manager/externs/background:media_scanner.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:crostini",
+    "//ui/file_manager/file_manager/externs/background:drive_sync_handler",
+    "//ui/file_manager/file_manager/externs/background:duplicate_finder",
+    "//ui/file_manager/file_manager/externs/background:file_browser_background_full",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
+    "//ui/file_manager/file_manager/externs/background:import_history",
+    "//ui/file_manager/file_manager/externs/background:media_import_handler",
+    "//ui/file_manager/file_manager/externs/background:media_scanner",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
   ]
 }
 
@@ -186,7 +186,7 @@ js_library("background_base") {
     ":volume_manager_factory",
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:background_base.m",
+    "//ui/file_manager/file_manager/externs/background:background_base",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:load_time_data.m",
   ]
@@ -196,7 +196,7 @@ js_library("crostini") {
   deps = [
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:crostini.m",
+    "//ui/file_manager/file_manager/externs/background:crostini",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:load_time_data.m",
   ]
@@ -206,7 +206,7 @@ js_library("mock_crostini") {
   testonly = true
   deps = [
     ":crostini",
-    "//ui/file_manager/file_manager/externs/background:crostini.m",
+    "//ui/file_manager/file_manager/externs/background:crostini",
   ]
   visibility += [
     "//ui/file_manager/file_manager/foreground/js:file_tasks_unittest.m",
@@ -222,7 +222,7 @@ js_unittest("crostini_unittest.m") {
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:entry_location.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:crostini.m",
+    "//ui/file_manager/file_manager/externs/background:crostini",
     "//ui/webui/resources/js:load_time_data.m",
   ]
 }
@@ -237,7 +237,7 @@ js_library("device_handler") {
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
     "//ui/webui/resources/js/cr:event_target.m",
   ]
 }
@@ -262,7 +262,7 @@ js_unittest("device_handler_unittest.m") {
 js_library("mock_drive_sync_handler") {
   deps = [
     "//ui/file_manager/file_manager/externs:drive_dialog_controller.m",
-    "//ui/file_manager/file_manager/externs/background:drive_sync_handler.m",
+    "//ui/file_manager/file_manager/externs/background:drive_sync_handler",
     "//ui/webui/resources/js/cr:event_target.m",
   ]
   visibility += [
@@ -278,8 +278,8 @@ js_library("drive_sync_handler") {
     "//ui/file_manager/file_manager/common/js:progress_center_common.m",
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/externs:drive_dialog_controller.m",
-    "//ui/file_manager/file_manager/externs/background:drive_sync_handler.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:drive_sync_handler",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
     "//ui/webui/resources/js/cr:event_target.m",
   ]
 }
@@ -304,8 +304,8 @@ js_library("duplicate_finder") {
     "//ui/file_manager/file_manager/common/js:metrics.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:duplicate_finder.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
+    "//ui/file_manager/file_manager/externs/background:duplicate_finder",
+    "//ui/file_manager/file_manager/externs/background:import_history",
   ]
 }
 
@@ -321,7 +321,7 @@ js_unittest("duplicate_finder_unittest.m") {
     "//ui/file_manager/file_manager/common/js:test_error_reporting.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
-    "//ui/file_manager/file_manager/externs/background:duplicate_finder.m",
+    "//ui/file_manager/file_manager/externs/background:duplicate_finder",
   ]
 }
 
@@ -339,8 +339,8 @@ js_library("file_operation_handler") {
     "//ui/file_manager/file_manager/common/js:file_operation_common.m",
     "//ui/file_manager/file_manager/common/js:progress_center_common.m",
     "//ui/file_manager/file_manager/common/js:util.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
   ]
 }
 
@@ -359,7 +359,7 @@ js_unittest("file_operation_handler_unittest.m") {
 
 js_library("mock_file_operation_manager") {
   deps = [
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js/cr:event_target.m",
   ]
@@ -379,7 +379,7 @@ js_library("file_operation_manager") {
     "//ui/file_manager/file_manager/externs:entry_location.m",
     "//ui/file_manager/file_manager/externs:files_app_entry_interfaces.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
     "//ui/webui/resources/js:assert.m",
   ]
 }
@@ -396,7 +396,7 @@ js_unittest("file_operation_manager_unittest.m") {
     "//ui/file_manager/file_manager/common/js:test_error_reporting.m",
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/externs:entry_location.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
     "//ui/webui/resources/js:load_time_data.m",
   ]
 }
@@ -435,7 +435,7 @@ js_library("import_history") {
     ":metadata_proxy",
     "//ui/file_manager/file_manager/common/js:importer_common.m",
     "//ui/file_manager/file_manager/common/js:util.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
+    "//ui/file_manager/file_manager/externs/background:import_history",
     "//ui/webui/resources/js:assert.m",
   ]
 }
@@ -450,7 +450,7 @@ js_unittest("import_history_unittest.m") {
     "//ui/file_manager/file_manager/common/js:test_error_reporting.m",
     "//ui/file_manager/file_manager/common/js:test_importer_common.m",
     "//ui/file_manager/file_manager/common/js:unittest_util.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
+    "//ui/file_manager/file_manager/externs/background:import_history",
   ]
 }
 
@@ -471,13 +471,13 @@ js_library("media_import_handler") {
     "//ui/file_manager/file_manager/common/js:progress_center_common.m",
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/common/js:xfm.m",
-    "//ui/file_manager/file_manager/externs/background:drive_sync_handler.m",
-    "//ui/file_manager/file_manager/externs/background:duplicate_finder.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
-    "//ui/file_manager/file_manager/externs/background:media_import_handler.m",
-    "//ui/file_manager/file_manager/externs/background:media_scanner.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
-    "//ui/file_manager/file_manager/externs/background:task_queue.m",
+    "//ui/file_manager/file_manager/externs/background:drive_sync_handler",
+    "//ui/file_manager/file_manager/externs/background:duplicate_finder",
+    "//ui/file_manager/file_manager/externs/background:import_history",
+    "//ui/file_manager/file_manager/externs/background:media_import_handler",
+    "//ui/file_manager/file_manager/externs/background:media_scanner",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
+    "//ui/file_manager/file_manager/externs/background:task_queue",
     "//ui/webui/resources/js:assert.m",
   ]
 }
@@ -499,9 +499,9 @@ js_unittest("media_import_handler_unittest.m") {
     "//ui/file_manager/file_manager/common/js:test_importer_common.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
-    "//ui/file_manager/file_manager/externs/background:duplicate_finder.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
-    "//ui/file_manager/file_manager/externs/background:media_import_handler.m",
+    "//ui/file_manager/file_manager/externs/background:duplicate_finder",
+    "//ui/file_manager/file_manager/externs/background:import_history",
+    "//ui/file_manager/file_manager/externs/background:media_import_handler",
   ]
 }
 
@@ -510,7 +510,7 @@ js_library("mock_media_scanner") {
     ":media_scanner",
     "//chrome/test/data/webui:chai_assert",
     "//ui/file_manager/file_manager/common/js:importer_common.m",
-    "//ui/file_manager/file_manager/externs/background:media_scanner.m",
+    "//ui/file_manager/file_manager/externs/background:media_scanner",
   ]
   visibility += [
     "//ui/file_manager/file_manager/foreground/js:import_controller_unittest.m",
@@ -522,7 +522,7 @@ js_library("media_scanner") {
     ":file_operation_util",
     ":metadata_proxy",
     "//ui/file_manager/file_manager/common/js:importer_common.m",
-    "//ui/file_manager/file_manager/externs/background:media_scanner.m",
+    "//ui/file_manager/file_manager/externs/background:media_scanner",
     "//ui/webui/resources/js:assert.m",
   ]
   visibility += [
@@ -540,7 +540,7 @@ js_unittest("media_scanner_unittest.m") {
     "//ui/file_manager/file_manager/common/js:importer_common.m",
     "//ui/file_manager/file_manager/common/js:test_error_reporting.m",
     "//ui/file_manager/file_manager/common/js:unittest_util.m",
-    "//ui/file_manager/file_manager/externs/background:media_scanner.m",
+    "//ui/file_manager/file_manager/externs/background:media_scanner",
   ]
 }
 
@@ -567,7 +567,7 @@ js_library("mock_volume_manager") {
 js_library("mock_progress_center") {
   deps = [
     "//ui/file_manager/file_manager/common/js:progress_center_common.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
   ]
   visibility +=
       [ "//ui/file_manager/file_manager/foreground/js:file_tasks_unittest.m" ]
@@ -579,7 +579,7 @@ js_library("progress_center") {
     "//ui/file_manager/file_manager/common/js:progress_center_common.m",
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/externs:progress_center_panel.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
   ]
 }
 
@@ -595,7 +595,7 @@ js_library("runtime_loaded_test_util") {
     "//ui/file_manager/file_manager/common/js:metrics.m",
     "//ui/file_manager/file_manager/common/js:progress_center_common.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
-    "//ui/file_manager/file_manager/externs/background:background_base.m",
+    "//ui/file_manager/file_manager/externs/background:background_base",
     "//ui/webui/resources/js:assert.m",
   ]
   externs_list = [ "$externs_path/webview_tag.js" ]
@@ -622,14 +622,14 @@ js_unittest("task_queue_unittest.m") {
     "//chrome/test/data/webui:chai_assert",
     "//ui/file_manager/file_manager/common/js:importer_common.m",
     "//ui/file_manager/file_manager/common/js:test_error_reporting.m",
-    "//ui/file_manager/file_manager/externs/background:task_queue.m",
+    "//ui/file_manager/file_manager/externs/background:task_queue",
   ]
 }
 
 js_library("task_queue") {
   deps = [
     "//ui/file_manager/file_manager/common/js:importer_common.m",
-    "//ui/file_manager/file_manager/externs/background:task_queue.m",
+    "//ui/file_manager/file_manager/externs/background:task_queue",
   ]
 }
 
diff --git a/ui/file_manager/file_manager/background/js/background.js b/ui/file_manager/file_manager/background/js/background.js
index 26ffb6f8471b9..bfb6b746ebb04 100644
--- a/ui/file_manager/file_manager/background/js/background.js
+++ b/ui/file_manager/file_manager/background/js/background.js
@@ -7,15 +7,15 @@ import {metrics} from '../../common/js/metrics.m.js';
 import {str, util} from '../../common/js/util.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
 import {xfm} from '../../common/js/xfm.m.js';
-import {Crostini} from '../../externs/background/crostini.m.js';
-import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.m.js';
-import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.m.js';
-import {FileBrowserBackgroundFull} from '../../externs/background/file_browser_background_full.m.js';
-import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
-import {importerHistoryInterfaces} from '../../externs/background/import_history.m.js';
-import {mediaImportInterfaces} from '../../externs/background/media_import_handler.m.js';
-import {mediaScannerInterfaces} from '../../externs/background/media_scanner.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+import {Crostini} from '../../externs/background/crostini.js';
+import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.js';
+import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.js';
+import {FileBrowserBackgroundFull} from '../../externs/background/file_browser_background_full.js';
+import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
+import {importerHistoryInterfaces} from '../../externs/background/import_history.js';
+import {mediaImportInterfaces} from '../../externs/background/media_import_handler.js';
+import {mediaScannerInterfaces} from '../../externs/background/media_scanner.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
 import {VolumeInfo} from '../../externs/volume_info.m.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 
diff --git a/ui/file_manager/file_manager/background/js/background_base.js b/ui/file_manager/file_manager/background/js/background_base.js
index 24fbc11a53f02..7c6f5ed6f2db3 100644
--- a/ui/file_manager/file_manager/background/js/background_base.js
+++ b/ui/file_manager/file_manager/background/js/background_base.js
@@ -6,7 +6,7 @@ import {assert} from 'chrome://resources/js/assert.m.js';
 import {loadTimeData} from 'chrome://resources/js/load_time_data.m.js';
 
 import {util} from '../../common/js/util.m.js';
-import {BackgroundBase, LaunchHandler} from '../../externs/background/background_base.m.js';
+import {BackgroundBase, LaunchHandler} from '../../externs/background/background_base.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 
 import {volumeManagerFactory} from './volume_manager_factory.js';
diff --git a/ui/file_manager/file_manager/background/js/crostini.js b/ui/file_manager/file_manager/background/js/crostini.js
index b85d1b02c1c85..3151014feb929 100644
--- a/ui/file_manager/file_manager/background/js/crostini.js
+++ b/ui/file_manager/file_manager/background/js/crostini.js
@@ -6,7 +6,7 @@ import {assert} from 'chrome://resources/js/assert.m.js';
 import {loadTimeData} from 'chrome://resources/js/load_time_data.m.js';
 
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
-import {Crostini} from '../../externs/background/crostini.m.js';
+import {Crostini} from '../../externs/background/crostini.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 
 /**
diff --git a/ui/file_manager/file_manager/background/js/crostini_unittest.m.js b/ui/file_manager/file_manager/background/js/crostini_unittest.m.js
index 1c605e4d357ad..4d3c75830e244 100644
--- a/ui/file_manager/file_manager/background/js/crostini_unittest.m.js
+++ b/ui/file_manager/file_manager/background/js/crostini_unittest.m.js
@@ -8,7 +8,7 @@ import { assertFalse,assertTrue} from 'chrome://test/chai_assert.js';
 import {installMockChrome} from '../../common/js/mock_chrome.m.js';
 import {MockDirectoryEntry, MockEntry, MockFileSystem} from '../../common/js/mock_entry.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
-import {Crostini} from '../../externs/background/crostini.m.js';
+import {Crostini} from '../../externs/background/crostini.js';
 import {EntryLocation} from '../../externs/entry_location.m.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 
diff --git a/ui/file_manager/file_manager/background/js/device_handler.js b/ui/file_manager/file_manager/background/js/device_handler.js
index 73662dd3eacbb..b0fd09104f7d6 100644
--- a/ui/file_manager/file_manager/background/js/device_handler.js
+++ b/ui/file_manager/file_manager/background/js/device_handler.js
@@ -10,7 +10,7 @@ import {metrics} from '../../common/js/metrics.m.js';
 import {ProgressCenterItem, ProgressItemState, ProgressItemType} from '../../common/js/progress_center_common.m.js';
 import {str, strf, util} from '../../common/js/util.m.js';
 import {xfm} from '../../common/js/xfm.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
 import {VolumeInfo} from '../../externs/volume_info.m.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 
diff --git a/ui/file_manager/file_manager/background/js/drive_sync_handler.js b/ui/file_manager/file_manager/background/js/drive_sync_handler.js
index 0a75738feb15c..600c4245dab20 100644
--- a/ui/file_manager/file_manager/background/js/drive_sync_handler.js
+++ b/ui/file_manager/file_manager/background/js/drive_sync_handler.js
@@ -8,8 +8,8 @@ import {AsyncUtil} from '../../common/js/async_util.m.js';
 import {ProgressCenterItem, ProgressItemState, ProgressItemType} from '../../common/js/progress_center_common.m.js';
 import {str, strf} from '../../common/js/util.m.js';
 import {xfm} from '../../common/js/xfm.m.js';
-import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
 import {DriveDialogControllerInterface} from '../../externs/drive_dialog_controller.m.js';
 
 import {fileOperationUtil} from './file_operation_util.js';
diff --git a/ui/file_manager/file_manager/background/js/duplicate_finder.js b/ui/file_manager/file_manager/background/js/duplicate_finder.js
index 7af382f9c6bb3..2956ba1c07c63 100644
--- a/ui/file_manager/file_manager/background/js/duplicate_finder.js
+++ b/ui/file_manager/file_manager/background/js/duplicate_finder.js
@@ -2,17 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-/**
- * @fileoverview
- * @suppress {uselessCode} Temporary suppress because of the line exporting.
- */
-
 import {importer} from '../../common/js/importer_common.m.js';
 import {LRUCache} from '../../common/js/lru_cache.m.js';
 import {metrics} from '../../common/js/metrics.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
-import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.m.js';
-import {importerHistoryInterfaces} from '../../externs/background/import_history.m.js';
+import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.js';
+import {importerHistoryInterfaces} from '../../externs/background/import_history.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 
 import {importerHistory} from './import_history.js';
@@ -289,5 +284,4 @@ duplicateFinder.DispositionCheckerImpl = class {
   }
 };
 
-// eslint-disable-next-line semi,no-extra-semi
 export {duplicateFinder};
diff --git a/ui/file_manager/file_manager/background/js/duplicate_finder_unittest.m.js b/ui/file_manager/file_manager/background/js/duplicate_finder_unittest.m.js
index 51d68d4835ab3..1ce8ae36c0fce 100644
--- a/ui/file_manager/file_manager/background/js/duplicate_finder_unittest.m.js
+++ b/ui/file_manager/file_manager/background/js/duplicate_finder_unittest.m.js
@@ -9,7 +9,7 @@ import {installMockChrome, MockCommandLinePrivate} from '../../common/js/mock_ch
 import {MockFileSystem} from '../../common/js/mock_entry.m.js';
 import {reportPromise} from '../../common/js/test_error_reporting.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
-import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.m.js';
+import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.js';
 import {VolumeInfo} from '../../externs/volume_info.m.js';
 
 import {duplicateFinder} from './duplicate_finder.js';
diff --git a/ui/file_manager/file_manager/background/js/file_operation_handler.js b/ui/file_manager/file_manager/background/js/file_operation_handler.js
index 578987c4ac351..a83e930d33b87 100644
--- a/ui/file_manager/file_manager/background/js/file_operation_handler.js
+++ b/ui/file_manager/file_manager/background/js/file_operation_handler.js
@@ -5,8 +5,8 @@
 import {FileOperationProgressEvent} from '../../common/js/file_operation_common.m.js';
 import {ProgressCenterItem, ProgressItemState, ProgressItemType} from '../../common/js/progress_center_common.m.js';
 import {str, strf, util} from '../../common/js/util.m.js';
-import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
 
 /**
  * An event handler of the background page for file operations.
diff --git a/ui/file_manager/file_manager/background/js/file_operation_manager.js b/ui/file_manager/file_manager/background/js/file_operation_manager.js
index c5bbc81633fe3..96810b68e56ae 100644
--- a/ui/file_manager/file_manager/background/js/file_operation_manager.js
+++ b/ui/file_manager/file_manager/background/js/file_operation_manager.js
@@ -9,7 +9,7 @@ import {FileOperationError, FileOperationProgressEvent} from '../../common/js/fi
 import {TrashEntry, TrashRootEntry} from '../../common/js/trash.js';
 import {util} from '../../common/js/util.m.js';
 import {xfm} from '../../common/js/xfm.m.js';
-import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
+import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
 import {EntryLocation} from '../../externs/entry_location.m.js';
 import {FakeEntry} from '../../externs/files_app_entry_interfaces.m.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
diff --git a/ui/file_manager/file_manager/background/js/file_operation_manager_unittest.m.js b/ui/file_manager/file_manager/background/js/file_operation_manager_unittest.m.js
index ff65adc21e667..d0b5c98c1fcaa 100644
--- a/ui/file_manager/file_manager/background/js/file_operation_manager_unittest.m.js
+++ b/ui/file_manager/file_manager/background/js/file_operation_manager_unittest.m.js
@@ -11,7 +11,7 @@ import {installMockChrome} from '../../common/js/mock_chrome.m.js';
 import { joinPath, MockDirectoryEntry, MockEntry, MockFileEntry,MockFileSystem} from '../../common/js/mock_entry.m.js';
 import {reportPromise, waitUntil} from '../../common/js/test_error_reporting.m.js';
 import {util} from '../../common/js/util.m.js';
-import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
+import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
 import {EntryLocation} from '../../externs/entry_location.m.js';
 
 import {FileOperationManagerImpl} from './file_operation_manager.js';
diff --git a/ui/file_manager/file_manager/background/js/import_history.js b/ui/file_manager/file_manager/background/js/import_history.js
index 5b525fd9eefbf..294b568e6a055 100644
--- a/ui/file_manager/file_manager/background/js/import_history.js
+++ b/ui/file_manager/file_manager/background/js/import_history.js
@@ -11,7 +11,7 @@ import {assertNotReached} from 'chrome://resources/js/assert.m.js';
 
 import {importer} from '../../common/js/importer_common.m.js';
 import {util} from '../../common/js/util.m.js';
-import {importerHistoryInterfaces} from '../../externs/background/import_history.m.js';
+import {importerHistoryInterfaces} from '../../externs/background/import_history.js';
 
 import {metadataProxy} from './metadata_proxy.js';
 
diff --git a/ui/file_manager/file_manager/background/js/import_history_unittest.m.js b/ui/file_manager/file_manager/background/js/import_history_unittest.m.js
index 066c24c02e9ae..0ab21be589e47 100644
--- a/ui/file_manager/file_manager/background/js/import_history_unittest.m.js
+++ b/ui/file_manager/file_manager/background/js/import_history_unittest.m.js
@@ -11,7 +11,7 @@ import {MockFileEntry,MockFileSystem} from '../../common/js/mock_entry.m.js';
 import {reportPromise} from '../../common/js/test_error_reporting.m.js';
 import {importerTest} from '../../common/js/test_importer_common.m.js';
 import {TestCallRecorder} from '../../common/js/unittest_util.m.js';
-import {importerHistoryInterfaces} from '../../externs/background/import_history.m.js';
+import {importerHistoryInterfaces} from '../../externs/background/import_history.js';
 
 import {importerHistory} from './import_history.js';
 // clang-format on
diff --git a/ui/file_manager/file_manager/background/js/media_import_handler.js b/ui/file_manager/file_manager/background/js/media_import_handler.js
index 2d9d2a4e5303a..2f2fe9eff0acb 100644
--- a/ui/file_manager/file_manager/background/js/media_import_handler.js
+++ b/ui/file_manager/file_manager/background/js/media_import_handler.js
@@ -10,13 +10,13 @@ import {metrics} from '../../common/js/metrics.m.js';
 import {ProgressCenterItem, ProgressItemState, ProgressItemType} from '../../common/js/progress_center_common.m.js';
 import {strf, util} from '../../common/js/util.m.js';
 import {xfm} from '../../common/js/xfm.m.js';
-import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.m.js';
-import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.m.js';
-import {importerHistoryInterfaces} from '../../externs/background/import_history.m.js';
-import {mediaImportInterfaces} from '../../externs/background/media_import_handler.m.js';
-import {mediaScannerInterfaces} from '../../externs/background/media_scanner.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
-import {taskQueueInterfaces} from '../../externs/background/task_queue.m.js';
+import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.js';
+import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.js';
+import {importerHistoryInterfaces} from '../../externs/background/import_history.js';
+import {mediaImportInterfaces} from '../../externs/background/media_import_handler.js';
+import {mediaScannerInterfaces} from '../../externs/background/media_scanner.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
+import {taskQueueInterfaces} from '../../externs/background/task_queue.js';
 
 import {fileOperationUtil} from './file_operation_util.js';
 import {taskQueue} from './task_queue.js';
diff --git a/ui/file_manager/file_manager/background/js/media_import_handler_unittest.m.js b/ui/file_manager/file_manager/background/js/media_import_handler_unittest.m.js
index df8eccafd53b4..dc465eca58dd7 100644
--- a/ui/file_manager/file_manager/background/js/media_import_handler_unittest.m.js
+++ b/ui/file_manager/file_manager/background/js/media_import_handler_unittest.m.js
@@ -11,9 +11,9 @@ import {reportPromise} from '../../common/js/test_error_reporting.m.js';
 import {importerTest} from '../../common/js/test_importer_common.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
 import {xfm} from '../../common/js/xfm.m.js';
-import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.m.js';
-import {importerHistoryInterfaces} from '../../externs/background/import_history.m.js';
-import {mediaImportInterfaces} from '../../externs/background/media_import_handler.m.js';
+import {duplicateFinderInterfaces} from '../../externs/background/duplicate_finder.js';
+import {importerHistoryInterfaces} from '../../externs/background/import_history.js';
+import {mediaImportInterfaces} from '../../externs/background/media_import_handler.js';
 import {VolumeInfo} from '../../externs/volume_info.m.js';
 
 import {fileOperationUtil} from './file_operation_util.js';
diff --git a/ui/file_manager/file_manager/background/js/media_scanner.js b/ui/file_manager/file_manager/background/js/media_scanner.js
index 70a827631ceeb..b0bd7d05c00f3 100644
--- a/ui/file_manager/file_manager/background/js/media_scanner.js
+++ b/ui/file_manager/file_manager/background/js/media_scanner.js
@@ -5,7 +5,7 @@
 import {assert} from 'chrome://resources/js/assert.m.js';
 
 import {importer} from '../../common/js/importer_common.m.js';
-import {mediaScannerInterfaces} from '../../externs/background/media_scanner.m.js';
+import {mediaScannerInterfaces} from '../../externs/background/media_scanner.js';
 
 import {fileOperationUtil} from './file_operation_util.js';
 import {metadataProxy} from './metadata_proxy.js';
diff --git a/ui/file_manager/file_manager/background/js/media_scanner_unittest.m.js b/ui/file_manager/file_manager/background/js/media_scanner_unittest.m.js
index 6cde7dddd37bc..c26a03e304f3d 100644
--- a/ui/file_manager/file_manager/background/js/media_scanner_unittest.m.js
+++ b/ui/file_manager/file_manager/background/js/media_scanner_unittest.m.js
@@ -7,7 +7,7 @@ import {assertEquals, assertFalse, assertThrows} from 'chrome://test/chai_assert
 import {importer} from '../../common/js/importer_common.m.js';
 import {reportPromise} from '../../common/js/test_error_reporting.m.js';
 import {assertFileEntryPathsEqual} from '../../common/js/unittest_util.m.js';
-import {mediaScannerInterfaces} from '../../externs/background/media_scanner.m.js';
+import {mediaScannerInterfaces} from '../../externs/background/media_scanner.js';
 
 import {fileOperationUtil} from './file_operation_util.js';
 import {mediaScanner} from './media_scanner.js';
diff --git a/ui/file_manager/file_manager/background/js/mock_crostini.js b/ui/file_manager/file_manager/background/js/mock_crostini.js
index 4253031c2327f..299073dcc67f7 100644
--- a/ui/file_manager/file_manager/background/js/mock_crostini.js
+++ b/ui/file_manager/file_manager/background/js/mock_crostini.js
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import {Crostini} from '../../externs/background/crostini.m.js';
+import {Crostini} from '../../externs/background/crostini.js';
 
 import {CrostiniImpl} from './crostini.js';
 
diff --git a/ui/file_manager/file_manager/background/js/mock_drive_sync_handler.js b/ui/file_manager/file_manager/background/js/mock_drive_sync_handler.js
index e3c1a391c122e..6c427e11e835d 100644
--- a/ui/file_manager/file_manager/background/js/mock_drive_sync_handler.js
+++ b/ui/file_manager/file_manager/background/js/mock_drive_sync_handler.js
@@ -4,7 +4,7 @@
 
 import {NativeEventTarget as EventTarget} from 'chrome://resources/js/cr/event_target.m.js';
 
-import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.m.js';
+import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.js';
 import {DriveDialogControllerInterface} from '../../externs/drive_dialog_controller.m.js';
 
 
diff --git a/ui/file_manager/file_manager/background/js/mock_file_operation_manager.js b/ui/file_manager/file_manager/background/js/mock_file_operation_manager.js
index 30939b2113d38..0f716c374e622 100644
--- a/ui/file_manager/file_manager/background/js/mock_file_operation_manager.js
+++ b/ui/file_manager/file_manager/background/js/mock_file_operation_manager.js
@@ -4,7 +4,7 @@
 
 import {assert} from 'chrome://resources/js/assert.m.js';
 import {NativeEventTarget as EventTarget} from 'chrome://resources/js/cr/event_target.m.js';
-import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
+import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
 
 
 /**
diff --git a/ui/file_manager/file_manager/background/js/mock_media_scanner.js b/ui/file_manager/file_manager/background/js/mock_media_scanner.js
index 244dce0b82cad..80587ea040506 100644
--- a/ui/file_manager/file_manager/background/js/mock_media_scanner.js
+++ b/ui/file_manager/file_manager/background/js/mock_media_scanner.js
@@ -5,7 +5,7 @@
 import {assertEquals, assertTrue} from 'chrome://test/chai_assert.js';
 
 import {importer} from '../../common/js/importer_common.m.js';
-import {mediaScannerInterfaces} from '../../externs/background/media_scanner.m.js';
+import {mediaScannerInterfaces} from '../../externs/background/media_scanner.js';
 
 import {mediaScanner} from './media_scanner.js';
 
diff --git a/ui/file_manager/file_manager/background/js/mock_progress_center.js b/ui/file_manager/file_manager/background/js/mock_progress_center.js
index 9e3f2854495e6..11a63451be727 100644
--- a/ui/file_manager/file_manager/background/js/mock_progress_center.js
+++ b/ui/file_manager/file_manager/background/js/mock_progress_center.js
@@ -4,7 +4,7 @@
 
 
 import {ProgressCenterItem, ProgressItemState} from '../../common/js/progress_center_common.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
 
 
 /**
diff --git a/ui/file_manager/file_manager/background/js/progress_center.js b/ui/file_manager/file_manager/background/js/progress_center.js
index 87c99856c120e..4d74eac9794c9 100644
--- a/ui/file_manager/file_manager/background/js/progress_center.js
+++ b/ui/file_manager/file_manager/background/js/progress_center.js
@@ -6,7 +6,7 @@ import {AsyncUtil} from '../../common/js/async_util.m.js';
 import {ProgressCenterItem, ProgressItemState} from '../../common/js/progress_center_common.m.js';
 import {str} from '../../common/js/util.m.js';
 import {xfm} from '../../common/js/xfm.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
 import {ProgressCenterPanelInterface} from '../../externs/progress_center_panel.m.js';
 
 /**
diff --git a/ui/file_manager/file_manager/background/js/runtime_loaded_test_util.js b/ui/file_manager/file_manager/background/js/runtime_loaded_test_util.js
index 8df957f15b880..dc44997afde6f 100644
--- a/ui/file_manager/file_manager/background/js/runtime_loaded_test_util.js
+++ b/ui/file_manager/file_manager/background/js/runtime_loaded_test_util.js
@@ -12,7 +12,7 @@ import {assert} from 'chrome://resources/js/assert.m.js';
 import {metrics} from '../../common/js/metrics.m.js';
 import {util} from '../../common/js/util.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
-import {BackgroundBase} from '../../externs/background/background_base.m.js';
+import {BackgroundBase} from '../../externs/background/background_base.js';
 import {test} from './test_util_base.js';
 
 /** @type {!BackgroundBase} */
diff --git a/ui/file_manager/file_manager/background/js/task_queue.js b/ui/file_manager/file_manager/background/js/task_queue.js
index 159f9ca1abde9..b6f84d2dc69ce 100644
--- a/ui/file_manager/file_manager/background/js/task_queue.js
+++ b/ui/file_manager/file_manager/background/js/task_queue.js
@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 import {importer} from '../../common/js/importer_common.m.js';
-import {taskQueueInterfaces} from '../../externs/background/task_queue.m.js';
+import {taskQueueInterfaces} from '../../externs/background/task_queue.js';
 
 // Namespace
 const taskQueue = {};
diff --git a/ui/file_manager/file_manager/background/js/task_queue_unittest.m.js b/ui/file_manager/file_manager/background/js/task_queue_unittest.m.js
index 0fe4fa3ee3c22..13a09d938dc29 100644
--- a/ui/file_manager/file_manager/background/js/task_queue_unittest.m.js
+++ b/ui/file_manager/file_manager/background/js/task_queue_unittest.m.js
@@ -6,7 +6,7 @@ import {assertEquals, assertFalse, assertTrue} from 'chrome://test/chai_assert.j
 
 import {importer} from '../../common/js/importer_common.m.js';
 import {reportPromise} from '../../common/js/test_error_reporting.m.js';
-import {taskQueueInterfaces} from '../../externs/background/task_queue.m.js';
+import {taskQueueInterfaces} from '../../externs/background/task_queue.js';
 
 import {taskQueue} from './task_queue.js';
 
diff --git a/ui/file_manager/file_manager/background/js/test_import_history.js b/ui/file_manager/file_manager/background/js/test_import_history.js
index 72adf46958f8f..0839eddefc597 100644
--- a/ui/file_manager/file_manager/background/js/test_import_history.js
+++ b/ui/file_manager/file_manager/background/js/test_import_history.js
@@ -2,14 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-/**
- * @fileoverview
- * @suppress {uselessCode} Temporary suppress because of the line exporting.
- */
-
 import {assertTrue} from 'chrome://test/chai_assert.js';
 import {importer} from '../../common/js/importer_common.m.js';
-import {importerHistoryInterfaces} from '../../externs/background/import_history.m.js';
+import {importerHistoryInterfaces} from '../../externs/background/import_history.js';
 
 // Namespace
 // eslint-disable-next-line no-var
@@ -156,5 +151,4 @@ importerTestHistory.TestImportHistory = class {
   removeObserver() {}
 };
 
-// eslint-disable-next-line semi,no-extra-semi
 export {importerTestHistory};
diff --git a/ui/file_manager/file_manager/common/js/BUILD.gn b/ui/file_manager/file_manager/common/js/BUILD.gn
index 24dcadc5aa99e..4f529d1480307 100644
--- a/ui/file_manager/file_manager/common/js/BUILD.gn
+++ b/ui/file_manager/file_manager/common/js/BUILD.gn
@@ -92,7 +92,7 @@ js_library("app_util.m") {
   deps = [
     "//ui/file_manager/file_manager/externs:file_manager_private",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:background_base.m",
+    "//ui/file_manager/file_manager/externs/background:background_base",
   ]
   externs_list =
       [ "//ui/file_manager/file_manager/externs/app_window_common.js" ]
diff --git a/ui/file_manager/file_manager/common/js/app_util.js b/ui/file_manager/file_manager/common/js/app_util.js
index eaafec79ae359..0525d668d259e 100644
--- a/ui/file_manager/file_manager/common/js/app_util.js
+++ b/ui/file_manager/file_manager/common/js/app_util.js
@@ -9,7 +9,7 @@
  */
 
 // clang-format off
-// #import {BackgroundBase} from '../../externs/background/background_base.m.js';
+// #import {BackgroundBase} from '../../externs/background/background_base.js';
 // #import {VolumeManager} from '../../externs/volume_manager.m.js';
 // #import {xfm} from './xfm.m.js';
 // clang-format on
diff --git a/ui/file_manager/file_manager/externs/BUILD.gn b/ui/file_manager/file_manager/externs/BUILD.gn
index deaa4bc4c7f4d..654da59948e0e 100644
--- a/ui/file_manager/file_manager/externs/BUILD.gn
+++ b/ui/file_manager/file_manager/externs/BUILD.gn
@@ -23,8 +23,7 @@ js_library("background_window.m") {
     "$root_gen_dir/ui/file_manager/file_manager/externs/background_window.m.js",
   ]
 
-  deps =
-      [ "//ui/file_manager/file_manager/externs/background:background_base.m" ]
+  deps = [ "//ui/file_manager/file_manager/externs/background:background_base" ]
 
   extra_deps = [ ":modulize" ]
 }
@@ -34,9 +33,9 @@ js_library("command_handler_deps.m") {
   deps = [
     ":files_app_entry_interfaces.m",
     ":volume_manager.m",
-    "background:crostini.m",
-    "background:file_operation_manager.m",
-    "background:progress_center.m",
+    "background:crostini",
+    "background:file_operation_manager",
+    "background:progress_center",
     "//ui/file_manager/file_manager/foreground/js:actions_controller.m",
     "//ui/file_manager/file_manager/foreground/js:dialog_type.m",
     "//ui/file_manager/file_manager/foreground/js:directory_contents.m",
diff --git a/ui/file_manager/file_manager/externs/background/BUILD.gn b/ui/file_manager/file_manager/externs/background/BUILD.gn
index 6666df13407cb..d8098e10c1224 100644
--- a/ui/file_manager/file_manager/externs/background/BUILD.gn
+++ b/ui/file_manager/file_manager/externs/background/BUILD.gn
@@ -3,147 +3,89 @@
 # found in the LICENSE file.
 
 import("//third_party/closure_compiler/compile_js.gni")
-import("//ui/webui/resources/tools/js_modulizer.gni")
+
+group("js_modules") {
+  deps = [
+    ":background_base",
+    ":crostini",
+    ":drive_sync_handler",
+    ":duplicate_finder",
+    ":file_browser_background_full",
+    ":file_operation_manager",
+    ":import_history",
+    ":media_import_handler",
+    ":media_scanner",
+    ":progress_center",
+    ":task_queue",
+  ]
+}
 
 js_library("file_browser_background_full") {
-  sources = []
-
-  externs_list = [
-    "background_base.js",
-    "crostini.js",
-    "../drive_dialog_controller.js",
-    "drive_sync_handler.js",
-    "duplicate_finder.js",
-    "file_browser_background_full.js",
-    "file_operation_manager.js",
-    "import_history.js",
-    "media_import_handler.js",
-    "media_scanner.js",
-    "progress_center.js",
-    "../progress_center_panel.js",
-    "task_queue.js",
-  ]
-}
-
-js_library("file_browser_background_full.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/file_browser_background_full.m.js" ]
   deps = [
-    ":background_base.m",
-    ":crostini.m",
-    ":drive_sync_handler.m",
-    ":file_operation_manager.m",
-    ":import_history.m",
-    ":media_import_handler.m",
-    ":media_scanner.m",
-    ":progress_center.m",
+    ":background_base",
+    ":crostini",
+    ":drive_sync_handler",
+    ":file_operation_manager",
+    ":import_history",
+    ":media_import_handler",
+    ":media_scanner",
+    ":progress_center",
   ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("background_base.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/background_base.m.js" ]
-
+js_library("background_base") {
   deps = [ "..:volume_manager.m" ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("drive_sync_handler.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/drive_sync_handler.m.js" ]
-
+js_library("drive_sync_handler") {
   deps = [ "..:drive_dialog_controller.m" ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("duplicate_finder.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/duplicate_finder.m.js" ]
+js_library("duplicate_finder") {
   deps = [
-    ":import_history.m",
+    ":import_history",
     "//ui/file_manager/file_manager/common/js:importer_common.m",
   ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("crostini.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/crostini.m.js" ]
+js_library("crostini") {
   deps = [ "..:volume_manager.m" ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("file_operation_manager.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/file_operation_manager.m.js" ]
+js_library("file_operation_manager") {
   deps = [
     "..:files_app_entry_interfaces.m",
     "..:volume_manager.m",
   ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("import_history.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/import_history.m.js" ]
+js_library("import_history") {
   deps = [ "//ui/file_manager/file_manager/common/js:importer_common.m" ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("media_import_handler.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/media_import_handler.m.js" ]
+js_library("media_import_handler") {
   deps = [
-    ":drive_sync_handler.m",
-    ":duplicate_finder.m",
-    ":import_history.m",
-    ":media_scanner.m",
-    ":progress_center.m",
-    ":task_queue.m",
+    ":drive_sync_handler",
+    ":duplicate_finder",
+    ":import_history",
+    ":media_scanner",
+    ":progress_center",
+    ":task_queue",
     "//ui/file_manager/file_manager/common/js:importer_common.m",
   ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("media_scanner.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/media_scanner.m.js" ]
+js_library("media_scanner") {
   deps = [ "//ui/file_manager/file_manager/common/js:importer_common.m" ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("progress_center.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/progress_center.m.js" ]
+js_library("progress_center") {
   deps = [
     "..:progress_center_panel.m",
     "//ui/file_manager/file_manager/common/js:progress_center_common.m",
   ]
-
-  extra_deps = [ ":modulize" ]
 }
 
-js_library("task_queue.m") {
-  sources = [ "$root_gen_dir/ui/file_manager/file_manager/externs/background/task_queue.m.js" ]
+js_library("task_queue") {
   deps = [ "//ui/file_manager/file_manager/common/js:importer_common.m" ]
-
-  extra_deps = [ ":modulize" ]
-}
-
-js_modulizer("modulize") {
-  input_files = [
-    "file_browser_background_full.js",
-    "background_base.js",
-    "crostini.js",
-    "drive_sync_handler.js",
-    "duplicate_finder.js",
-    "file_operation_manager.js",
-    "import_history.js",
-    "media_import_handler.js",
-    "media_scanner.js",
-    "progress_center.js",
-    "task_queue.js",
-  ]
-
-  preserve_offsets = [ "True" ]
 }
diff --git a/ui/file_manager/file_manager/externs/background/background_base.js b/ui/file_manager/file_manager/externs/background/background_base.js
index 6fc0b47f49b48..c9c1caac00350 100644
--- a/ui/file_manager/file_manager/externs/background/background_base.js
+++ b/ui/file_manager/file_manager/externs/background/background_base.js
@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// #import {VolumeManager} from '../volume_manager.m.js';
+import {VolumeManager} from '../volume_manager.m.js';
 
 /** @typedef {function(!Array<string>):!Promise} */
-/* #export */ let LaunchHandler;
+export let LaunchHandler;
 
 /**
  * Interface exposed in window.background in the background page. Used for
@@ -19,7 +19,7 @@
  *
  * @interface
  */
-/* #export */ class BackgroundBase {
+export class BackgroundBase {
   constructor() {
     /** @type {!Object<!Window>} */
     this.dialogs;
diff --git a/ui/file_manager/file_manager/externs/background/crostini.js b/ui/file_manager/file_manager/externs/background/crostini.js
index 3f9e434476e27..7d47f0122c25d 100644
--- a/ui/file_manager/file_manager/externs/background/crostini.js
+++ b/ui/file_manager/file_manager/externs/background/crostini.js
@@ -2,14 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// #import {VolumeManager} from '../volume_manager.m.js';
+import {VolumeManager} from '../volume_manager.m.js';
 
 /**
  * Crostini shared path state handler.
  *
  * @interface
  */
-/* #export */ class Crostini {
+export class Crostini {
   /**
    * Initialize enabled settings and register for any shared path changes.
    * Must be done after loadTimeData is available.
diff --git a/ui/file_manager/file_manager/externs/background/drive_sync_handler.js b/ui/file_manager/file_manager/externs/background/drive_sync_handler.js
index 218437c92659e..35492ef784065 100644
--- a/ui/file_manager/file_manager/externs/background/drive_sync_handler.js
+++ b/ui/file_manager/file_manager/externs/background/drive_sync_handler.js
@@ -2,9 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// clang-format off
-// #import {DriveDialogControllerInterface} from '../drive_dialog_controller.m.js';
-// clang-format on
+import {DriveDialogControllerInterface} from '../drive_dialog_controller.m.js';
 
 /**
  * Handler of the background page for the Drive sync events. Implementations
@@ -12,7 +10,7 @@
  *
  * @interface
  */
-/* #export */ class DriveSyncHandler extends EventTarget {
+export class DriveSyncHandler extends EventTarget {
   /**
    * Returns the completed event name.
    * @return {string}
diff --git a/ui/file_manager/file_manager/externs/background/duplicate_finder.js b/ui/file_manager/file_manager/externs/background/duplicate_finder.js
index bfaaafe1f5b3b..323c00042b228 100644
--- a/ui/file_manager/file_manager/externs/background/duplicate_finder.js
+++ b/ui/file_manager/file_manager/externs/background/duplicate_finder.js
@@ -2,11 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// #import {importer} from '../../common/js/importer_common.m.js';
-// #import {importerHistoryInterfaces} from './import_history.m.js';
+import {importer} from '../../common/js/importer_common.m.js';
+import {importerHistoryInterfaces} from './import_history.js';
 
 // Namespace
-/* #export */ const duplicateFinderInterfaces = {};
+export const duplicateFinderInterfaces = {};
 
 /**
  * Declare DispositionChecker class.
diff --git a/ui/file_manager/file_manager/externs/background/file_browser_background_full.js b/ui/file_manager/file_manager/externs/background/file_browser_background_full.js
index efa0f34e59302..2fce42d953b56 100644
--- a/ui/file_manager/file_manager/externs/background/file_browser_background_full.js
+++ b/ui/file_manager/file_manager/externs/background/file_browser_background_full.js
@@ -2,25 +2,19 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-/**
- * @fileoverview
- * @suppress {externsValidation} this file is used as externs and also
- * as JS module, Closure fails to compile as JS module.
- */
-
-// #import {DriveSyncHandler} from './drive_sync_handler.m.js';
-// #import {ProgressCenter} from './progress_center.m.js';
-// #import {FileOperationManager} from './file_operation_manager.m.js';
-// #import {mediaImportInterfaces} from './media_import_handler.m.js';
-// #import {mediaScannerInterfaces} from './media_scanner.m.js';
-// #import {importerHistoryInterfaces} from './import_history.m.js';
-// #import {Crostini} from './crostini.m.js';
-// #import {BackgroundBase} from './background_base.m.js';
+import {BackgroundBase} from './background_base.js';
+import {Crostini} from './crostini.js';
+import {DriveSyncHandler} from './drive_sync_handler.js';
+import {FileOperationManager} from './file_operation_manager.js';
+import {importerHistoryInterfaces} from './import_history.js';
+import {mediaImportInterfaces} from './media_import_handler.js';
+import {mediaScannerInterfaces} from './media_scanner.js';
+import {ProgressCenter} from './progress_center.js';
 
 /**
  * @interface
  */
-/* #export */ class FileBrowserBackgroundFull extends BackgroundBase {
+export class FileBrowserBackgroundFull extends BackgroundBase {
   constructor() {
     super();
 
diff --git a/ui/file_manager/file_manager/externs/background/file_operation_manager.js b/ui/file_manager/file_manager/externs/background/file_operation_manager.js
index 0b20eb64a6582..bc6f60ccab4e9 100644
--- a/ui/file_manager/file_manager/externs/background/file_operation_manager.js
+++ b/ui/file_manager/file_manager/externs/background/file_operation_manager.js
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// #import {VolumeManager} from '../volume_manager.m.js';
-// #import {FilesAppEntry, FakeEntry} from '../files_app_entry_interfaces.m.js';
+import {FakeEntry, FilesAppEntry} from '../files_app_entry_interfaces.m.js';
+import {VolumeManager} from '../volume_manager.m.js';
 
 /**
  * FileOperationManager: manager of file operations. Implementations of this
@@ -12,7 +12,7 @@
  *
  * @interface
  */
-/* #export */ class FileOperationManager extends EventTarget {
+export class FileOperationManager extends EventTarget {
   /**
    * Says if there are any tasks in the queue.
    * @return {boolean} True, if there are any tasks.
diff --git a/ui/file_manager/file_manager/externs/background/import_history.js b/ui/file_manager/file_manager/externs/background/import_history.js
index e1f671fe7073b..6eec65aa0cd12 100644
--- a/ui/file_manager/file_manager/externs/background/import_history.js
+++ b/ui/file_manager/file_manager/externs/background/import_history.js
@@ -2,13 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-/**
- * @fileoverview
- * @suppress {uselessCode, externsValidation} Temporary suppress because of the
- * line exporting.
- */
-
-// #import {importer} from '../../common/js/importer_common.m.js';
+import {importer} from '../../common/js/importer_common.m.js';
 
 // Namespace
 // eslint-disable-next-line no-var
@@ -139,5 +133,4 @@ importerHistoryInterfaces.HistoryLoader = class {
   addHistoryLoadedListener(listener) {}
 };
 
-// eslint-disable-next-line semi,no-extra-semi
-/* #export */ {importerHistoryInterfaces};
+export {importerHistoryInterfaces};
diff --git a/ui/file_manager/file_manager/externs/background/media_import_handler.js b/ui/file_manager/file_manager/externs/background/media_import_handler.js
index 3794402ccfce8..e70e62aa485e0 100644
--- a/ui/file_manager/file_manager/externs/background/media_import_handler.js
+++ b/ui/file_manager/file_manager/externs/background/media_import_handler.js
@@ -8,15 +8,16 @@
  * of its media import unittest and caller sites.
  */
 
-// #import {DriveSyncHandler} from './drive_sync_handler.m.js';
-// #import {importer} from '../../common/js/importer_common.m.js';
-// #import {importerHistoryInterfaces} from './import_history.m.js';
-// #import {duplicateFinderInterfaces} from './duplicate_finder.m.js';
-// #import {mediaScannerInterfaces} from './media_scanner.m.js';
-// #import {taskQueueInterfaces} from './task_queue.m.js';
-// #import {ProgressCenter} from './progress_center.m.js';
+import {importer} from '../../common/js/importer_common.m.js';
 
-/* #export */ const mediaImportInterfaces = {};
+import {DriveSyncHandler} from './drive_sync_handler.js';
+import {duplicateFinderInterfaces} from './duplicate_finder.js';
+import {importerHistoryInterfaces} from './import_history.js';
+import {mediaScannerInterfaces} from './media_scanner.js';
+import {ProgressCenter} from './progress_center.js';
+import {taskQueueInterfaces} from './task_queue.js';
+
+export const mediaImportInterfaces = {};
 
 /**
  * Interface providing access to information about active import processes.
diff --git a/ui/file_manager/file_manager/externs/background/media_scanner.js b/ui/file_manager/file_manager/externs/background/media_scanner.js
index 23aa571cedf9d..57feeca8f3778 100644
--- a/ui/file_manager/file_manager/externs/background/media_scanner.js
+++ b/ui/file_manager/file_manager/externs/background/media_scanner.js
@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// #import {importer} from '../../common/js/importer_common.m.js';
+import {importer} from '../../common/js/importer_common.m.js';
 
-/* #export */ const mediaScannerInterfaces = {};
+export const mediaScannerInterfaces = {};
 /**
  * Class representing the results of a scan operation.
  *
diff --git a/ui/file_manager/file_manager/externs/background/progress_center.js b/ui/file_manager/file_manager/externs/background/progress_center.js
index f799d63ac7427..faa94bb053613 100644
--- a/ui/file_manager/file_manager/externs/background/progress_center.js
+++ b/ui/file_manager/file_manager/externs/background/progress_center.js
@@ -2,16 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// clang-format off
-// #import {ProgressCenterItem} from '../../common/js/progress_center_common.m.js';
-// #import {ProgressCenterPanelInterface} from '../progress_center_panel.m.js';
-// clang-format on
+import {ProgressCenterItem} from '../../common/js/progress_center_common.m.js';
+import {ProgressCenterPanelInterface} from '../progress_center_panel.m.js';
 
 /**
  * Progress center at the background page.
  * @interface
  */
-/* #export */ class ProgressCenter {
+export class ProgressCenter {
   /**
    * Turns off sending updates when a file operation reaches 'completed' state.
    * Used for testing UI that can be ephemeral otherwise.
diff --git a/ui/file_manager/file_manager/externs/background/task_queue.js b/ui/file_manager/file_manager/externs/background/task_queue.js
index ed12ecf567193..35b082689b099 100644
--- a/ui/file_manager/file_manager/externs/background/task_queue.js
+++ b/ui/file_manager/file_manager/externs/background/task_queue.js
@@ -2,15 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-/**
- * @fileoverview
- * @suppress {uselessCode, externsValidation} Temporary suppress because of the
- * line exporting.
- */
-
-// clang-format off
-// #import {importer} from '../../common/js/importer_common.m.js';
-// clang-format on
+import {importer} from '../../common/js/importer_common.m.js';
 
 const taskQueueInterfaces = {};
 
@@ -104,5 +96,4 @@ taskQueueInterfaces.BaseTask = class extends taskQueueInterfaces.Task {
  */
 taskQueueInterfaces.Task.Observer;
 
-// eslint-disable-next-line semi,no-extra-semi
-/* #export */ {taskQueueInterfaces};
+export {taskQueueInterfaces};
diff --git a/ui/file_manager/file_manager/externs/background_window.js b/ui/file_manager/file_manager/externs/background_window.js
index 410d8fce168c8..5a61a1466f2b2 100644
--- a/ui/file_manager/file_manager/externs/background_window.js
+++ b/ui/file_manager/file_manager/externs/background_window.js
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// #import {BackgroundBase} from './background/background_base.m.js';
+// #import {BackgroundBase} from './background/background_base.js';
 
 /**
  * @extends {Window}
diff --git a/ui/file_manager/file_manager/externs/command_handler_deps.js b/ui/file_manager/file_manager/externs/command_handler_deps.js
index da3b3720047c6..60940783767f0 100644
--- a/ui/file_manager/file_manager/externs/command_handler_deps.js
+++ b/ui/file_manager/file_manager/externs/command_handler_deps.js
@@ -5,18 +5,18 @@
 // clang-format off
 // #import {ActionsController} from '../foreground/js/actions_controller.m.js';
 // #import {FilesAppEntry} from './files_app_entry_interfaces.m.js';
-// #import {Crostini} from './background/crostini.m.js';
+// #import {Crostini} from './background/crostini.js';
 // #import {MetadataModel} from '../foreground/js/metadata/metadata_model.js';
 // #import {VolumeManager} from './volume_manager.m.js';
 // #import {FileManagerUI} from '../foreground/js/ui/file_manager_ui.js';
 // #import {TaskController} from '../foreground/js/task_controller.m.js';
 // #import {SpinnerController} from '../foreground/js/spinner_controller.m.js';
 // #import {ProvidersModel} from '../foreground/js/providers_model.m.js';
-// #import {ProgressCenter} from './background/progress_center.m.js';
+// #import {ProgressCenter} from './background/progress_center.js';
 // #import {NamingController} from '../foreground/js/naming_controller.m.js';
 // #import {FileSelectionHandler, FileSelection} from '../foreground/js/file_selection.m.js';
 // #import {FileTransferController} from '../foreground/js/file_transfer_controller.m.js';
-// #import {FileOperationManager} from './background/file_operation_manager.m.js';
+// #import {FileOperationManager} from './background/file_operation_manager.js';
 // #import {FileFilter} from '../foreground/js/directory_contents.m.js';
 // #import {DirectoryTreeNamingController} from '../foreground/js/directory_tree_naming_controller.m.js';
 // #import {DirectoryTree} from '../foreground/js/ui/directory_tree.js';
diff --git a/ui/file_manager/file_manager/foreground/js/BUILD.gn b/ui/file_manager/file_manager/foreground/js/BUILD.gn
index 7991c8186a4f8..13d4c7c811d02 100644
--- a/ui/file_manager/file_manager/foreground/js/BUILD.gn
+++ b/ui/file_manager/file_manager/foreground/js/BUILD.gn
@@ -119,7 +119,7 @@ js_library("actions_controller.m") {
     "metadata:metadata_model",
     "ui:file_manager_ui",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:drive_sync_handler.m",
+    "//ui/file_manager/file_manager/externs/background:drive_sync_handler",
     "//ui/webui/resources/js/cr/ui:context_menu_handler.m",
   ]
 
@@ -147,7 +147,7 @@ js_library("actions_model.m") {
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:drive_sync_handler.m",
+    "//ui/file_manager/file_manager/externs/background:drive_sync_handler",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:cr.m",
     "//ui/webui/resources/js/cr:event_target.m",
@@ -397,7 +397,7 @@ js_library("crostini_controller.m") {
     "//ui/file_manager/file_manager/common/js:files_app_entry_types.m",
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
-    "//ui/file_manager/file_manager/externs/background:crostini.m",
+    "//ui/file_manager/file_manager/externs/background:crostini",
     "//ui/file_manager/file_manager/foreground/elements:files_message",
     "//ui/file_manager/file_manager/foreground/elements:files_toast",
     "//ui/webui/resources/js:assert.m",
@@ -526,7 +526,7 @@ js_library("directory_model.m") {
     "//ui/file_manager/file_manager/externs:files_app_entry_interfaces.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:cr.m",
     "//ui/webui/resources/js/cr:event_target.m",
@@ -788,13 +788,13 @@ js_library("file_manager.m") {
     "//ui/file_manager/file_manager/externs:background_window.m",
     "//ui/file_manager/file_manager/externs:command_handler_deps.m",
     "//ui/file_manager/file_manager/externs:files_app_entry_interfaces.m",
-    "//ui/file_manager/file_manager/externs/background:crostini.m",
-    "//ui/file_manager/file_manager/externs/background:file_browser_background_full.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
-    "//ui/file_manager/file_manager/externs/background:media_import_handler.m",
-    "//ui/file_manager/file_manager/externs/background:media_scanner.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:crostini",
+    "//ui/file_manager/file_manager/externs/background:file_browser_background_full",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
+    "//ui/file_manager/file_manager/externs/background:import_history",
+    "//ui/file_manager/file_manager/externs/background:media_import_handler",
+    "//ui/file_manager/file_manager/externs/background:media_scanner",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
     "//ui/file_manager/file_manager/foreground/elements:files_message",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:load_time_data.m",
@@ -908,7 +908,7 @@ js_library("file_selection.m") {
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:cr.m",
     "//ui/webui/resources/js/cr:event_target.m",
@@ -956,8 +956,8 @@ js_library("file_tasks.m") {
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:crostini.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:crostini",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
     "//ui/file_manager/file_manager/foreground/elements:files_password_dialog",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:load_time_data.m",
@@ -987,7 +987,7 @@ js_unittest("file_tasks_unittest.m") {
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:entry_location.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
     "//ui/file_manager/file_manager/foreground/elements:files_password_dialog",
   ]
 }
@@ -1025,8 +1025,8 @@ js_library("file_transfer_controller.m") {
     "//ui/file_manager/file_manager/externs:files_app_entry_interfaces.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:util.m",
     "//ui/webui/resources/js/cr/ui:command.m",
@@ -1058,9 +1058,9 @@ js_unittest("file_transfer_controller_unittest.m") {
     "//ui/file_manager/file_manager/common/js:mock_entry.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
+    "//ui/file_manager/file_manager/externs/background:import_history",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
     "//ui/webui/resources/js:util.m",
     "//ui/webui/resources/js/cr:ui.m",
     "//ui/webui/resources/js/cr/ui:command.m",
@@ -1222,8 +1222,8 @@ js_library("import_controller.m") {
     "//ui/file_manager/file_manager/externs:files_app_entry_interfaces.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:media_import_handler.m",
-    "//ui/file_manager/file_manager/externs/background:media_scanner.m",
+    "//ui/file_manager/file_manager/externs/background:media_import_handler",
+    "//ui/file_manager/file_manager/externs/background:media_scanner",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:cr.m",
     "//ui/webui/resources/js:util.m",
@@ -1244,8 +1244,8 @@ js_unittest("import_controller_unittest.m") {
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:media_import_handler.m",
-    "//ui/file_manager/file_manager/externs/background:media_scanner.m",
+    "//ui/file_manager/file_manager/externs/background:media_import_handler",
+    "//ui/file_manager/file_manager/externs/background:media_scanner",
     "//ui/webui/resources/js:assert.m",
   ]
 }
@@ -1765,8 +1765,8 @@ js_library("task_controller.m") {
     "ui:file_manager_ui",
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:crostini.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:crostini",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js:load_time_data.m",
     "//ui/webui/resources/js/cr/ui:command.m",
@@ -1795,7 +1795,7 @@ js_unittest("task_controller_unittest.m") {
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:progress_center.m",
+    "//ui/file_manager/file_manager/externs/background:progress_center",
     "//ui/webui/resources/js:assert.m",
     "//ui/webui/resources/js/cr:ui.m",
     "//ui/webui/resources/js/cr/ui:command.m",
diff --git a/ui/file_manager/file_manager/foreground/js/actions_controller.js b/ui/file_manager/file_manager/foreground/js/actions_controller.js
index 9694f2488f0a7..a54fc4bd697a1 100644
--- a/ui/file_manager/file_manager/foreground/js/actions_controller.js
+++ b/ui/file_manager/file_manager/foreground/js/actions_controller.js
@@ -4,7 +4,7 @@
 
 // clang-format off
 // #import {FileManagerUI} from './ui/file_manager_ui.js';
-// #import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.m.js';
+// #import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.js';
 // #import {FolderShortcutsDataModel} from './folder_shortcuts_data_model.m.js';
 // #import {DirectoryModel} from './directory_model.m.js';
 // #import {MetadataModel} from './metadata/metadata_model.js';
diff --git a/ui/file_manager/file_manager/foreground/js/actions_model.js b/ui/file_manager/file_manager/foreground/js/actions_model.js
index 65282ad121b01..fcbf8da32523f 100644
--- a/ui/file_manager/file_manager/foreground/js/actions_model.js
+++ b/ui/file_manager/file_manager/foreground/js/actions_model.js
@@ -5,7 +5,7 @@
 // clang-format off
 // #import {ActionModelUI} from './ui/action_model_ui.js';
 // #import {FolderShortcutsDataModel} from './folder_shortcuts_data_model.m.js';
-// #import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.m.js';
+// #import {DriveSyncHandler} from '../../externs/background/drive_sync_handler.js';
 // #import {VolumeManager} from '../../externs/volume_manager.m.js';
 // #import {MetadataModel} from './metadata/metadata_model.js';
 // #import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/crostini_controller.js b/ui/file_manager/file_manager/foreground/js/crostini_controller.js
index 84f51f0ef563f..3de3d4bf033ad 100644
--- a/ui/file_manager/file_manager/foreground/js/crostini_controller.js
+++ b/ui/file_manager/file_manager/foreground/js/crostini_controller.js
@@ -7,7 +7,7 @@
 // #import {DirectoryTree} from './ui/directory_tree.js';
 // #import {DirectoryModel} from './directory_model.m.js';
 // #import {FilesMessage} from '../elements/files_message.js';
-// #import {Crostini} from '../../externs/background/crostini.m.js';
+// #import {Crostini} from '../../externs/background/crostini.js';
 // #import {CommandHandler} from './file_manager_commands.m.js';
 // #import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
 // #import {FakeEntryImpl} from '../../common/js/files_app_entry_types.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/directory_model.js b/ui/file_manager/file_manager/foreground/js/directory_model.js
index 3347fd66f5f47..894b6e8966e2f 100644
--- a/ui/file_manager/file_manager/foreground/js/directory_model.js
+++ b/ui/file_manager/file_manager/foreground/js/directory_model.js
@@ -8,7 +8,7 @@
 // #import {ListSelectionModel} from 'chrome://resources/js/cr/ui/list_selection_model.m.js';
 // #import {VolumeInfo} from '../../externs/volume_info.m.js';
 // #import {FilesAppDirEntry, FakeEntry, FilesAppEntry} from '../../externs/files_app_entry_interfaces.m.js';
-// #import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
+// #import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
 // #import {VolumeManager} from '../../externs/volume_manager.m.js';
 // #import {MetadataModel} from './metadata/metadata_model.js';
 // #import {FileListSingleSelectionModel, FileListSelectionModel} from './ui/file_list_selection_model.js';
diff --git a/ui/file_manager/file_manager/foreground/js/file_manager.js b/ui/file_manager/file_manager/foreground/js/file_manager.js
index 88f39f30056a0..58d05f18983a6 100644
--- a/ui/file_manager/file_manager/foreground/js/file_manager.js
+++ b/ui/file_manager/file_manager/foreground/js/file_manager.js
@@ -8,15 +8,15 @@
 // #import {FilesMessage} from '../elements/files_message.js';
 // #import {FileListSelectionModel} from './ui/file_list_selection_model.js';
 // #import {A11yAnnounce} from './ui/a11y_announce.js';
-// #import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+// #import {ProgressCenter} from '../../externs/background/progress_center.js';
 // #import {FakeEntry, FilesAppDirEntry} from '../../externs/files_app_entry_interfaces.m.js';
-// #import {FileBrowserBackgroundFull} from '../../externs/background/file_browser_background_full.m.js';
+// #import {FileBrowserBackgroundFull} from '../../externs/background/file_browser_background_full.js';
 // #import {BackgroundWindow} from '../../externs/background_window.m.js';
-// #import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
-// #import {mediaImportInterfaces} from '../../externs/background/media_import_handler.m.js';
-// #import {mediaScannerInterfaces} from '../../externs/background/media_scanner.m.js';
-// #import {Crostini} from '../../externs/background/crostini.m.js';
-// #import {importerHistoryInterfaces} from '../../externs/background/import_history.m.js';
+// #import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
+// #import {mediaImportInterfaces} from '../../externs/background/media_import_handler.js';
+// #import {mediaScannerInterfaces} from '../../externs/background/media_scanner.js';
+// #import {Crostini} from '../../externs/background/crostini.js';
+// #import {importerHistoryInterfaces} from '../../externs/background/import_history.js';
 // #import {CommandHandlerDeps} from '../../externs/command_handler_deps.m.js';
 // #import {ProgressItemState} from '../../common/js/progress_center_common.m.js';
 // #import {xfm} from '../../common/js/xfm.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/file_selection.js b/ui/file_manager/file_manager/foreground/js/file_selection.js
index 9eaf5cf124d5a..421d3cdc64804 100644
--- a/ui/file_manager/file_manager/foreground/js/file_selection.js
+++ b/ui/file_manager/file_manager/foreground/js/file_selection.js
@@ -5,7 +5,7 @@
 // clang-format off
 // #import {MetadataModel} from './metadata/metadata_model.js';
 // #import {ListContainer} from './ui/list_container.js';
-// #import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
+// #import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
 // #import {DirectoryModel} from './directory_model.m.js';
 // #import {VolumeManager} from '../../externs/volume_manager.m.js';
 // #import {AllowedPaths} from '../../common/js/volume_manager_types.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/file_tasks.js b/ui/file_manager/file_manager/foreground/js/file_tasks.js
index 3ce19d44c528c..b96734aa4d6f5 100644
--- a/ui/file_manager/file_manager/foreground/js/file_tasks.js
+++ b/ui/file_manager/file_manager/foreground/js/file_tasks.js
@@ -6,8 +6,8 @@
 // #import {Menu} from 'chrome://resources/js/cr/ui/menu.m.js';
 // #import {MultiMenuButton} from './ui/multi_menu_button.js';
 // #import {VolumeInfo} from '../../externs/volume_info.m.js';
-// #import {ProgressCenter} from '../../externs/background/progress_center.m.js';
-// #import {Crostini} from '../../externs/background/crostini.m.js';
+// #import {ProgressCenter} from '../../externs/background/progress_center.js';
+// #import {Crostini} from '../../externs/background/crostini.js';
 // #import {NamingController} from './naming_controller.m.js';
 // #import {TaskHistory} from './task_history.m.js';
 // #import {FileManagerUI} from './ui/file_manager_ui.js';
diff --git a/ui/file_manager/file_manager/foreground/js/file_tasks_unittest.m.js b/ui/file_manager/file_manager/foreground/js/file_tasks_unittest.m.js
index 90e8548b51c11..fb4e311e68a28 100644
--- a/ui/file_manager/file_manager/foreground/js/file_tasks_unittest.m.js
+++ b/ui/file_manager/file_manager/foreground/js/file_tasks_unittest.m.js
@@ -12,7 +12,7 @@ import {MockFileEntry, MockFileSystem} from '../../common/js/mock_entry.m.js';
 import {ProgressItemState} from '../../common/js/progress_center_common.m.js';
 import {reportPromise} from '../../common/js/test_error_reporting.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
 import {EntryLocation} from '../../externs/entry_location.m.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 import {FilesPasswordDialog} from '../elements/files_password_dialog.js';
diff --git a/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js b/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js
index 1fecc0a8694b3..d9c95c16821d3 100644
--- a/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js
+++ b/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js
@@ -11,8 +11,8 @@
 // #import {VolumeManager} from '../../externs/volume_manager.m.js';
 // #import {DirectoryModel} from './directory_model.m.js';
 // #import {MetadataModel} from './metadata/metadata_model.js';
-// #import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
-// #import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+// #import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
+// #import {ProgressCenter} from '../../externs/background/progress_center.js';
 // #import {ListContainer} from './ui/list_container.js';
 // #import {DropEffectAndLabel, DropEffectType} from './drop_effect_and_label.m.js';
 // #import {FileSelectionHandler} from './file_selection.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/file_transfer_controller_unittest.m.js b/ui/file_manager/file_manager/foreground/js/file_transfer_controller_unittest.m.js
index ed08ce703d5c4..b787d7147c9c5 100644
--- a/ui/file_manager/file_manager/foreground/js/file_transfer_controller_unittest.m.js
+++ b/ui/file_manager/file_manager/foreground/js/file_transfer_controller_unittest.m.js
@@ -12,9 +12,9 @@ import {MockVolumeManager} from '../../background/js/mock_volume_manager.js';
 import {installMockChrome} from '../../common/js/mock_chrome.m.js';
 import {MockDirectoryEntry, MockFileEntry, MockFileSystem} from '../../common/js/mock_entry.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
-import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
-import {importerHistoryInterfaces} from '../../externs/background/import_history.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
+import {importerHistoryInterfaces} from '../../externs/background/import_history.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 
 import {DialogType} from './dialog_type.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/import_controller.js b/ui/file_manager/file_manager/foreground/js/import_controller.js
index 8581d8a6f4445..8d0894fadd3c3 100644
--- a/ui/file_manager/file_manager/foreground/js/import_controller.js
+++ b/ui/file_manager/file_manager/foreground/js/import_controller.js
@@ -13,8 +13,8 @@
 // #import {VolumeInfo} from '../../externs/volume_info.m.js';
 // #import {FilesAppEntry} from '../../externs/files_app_entry_interfaces.m.js';
 // #import {VolumeManager} from '../../externs/volume_manager.m.js';
-// #import {mediaImportInterfaces} from '../../externs/background/media_import_handler.m.js';
-// #import {mediaScannerInterfaces} from '../../externs/background/media_scanner.m.js';
+// #import {mediaImportInterfaces} from '../../externs/background/media_import_handler.js';
+// #import {mediaScannerInterfaces} from '../../externs/background/media_scanner.js';
 // #import {FileSelectionHandler} from './file_selection.m.js';
 // #import {util, strf, str} from '../../common/js/util.m.js';
 // #import {queryRequiredElement, getRequiredElement} from 'chrome://resources/js/util.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/import_controller_unittest.m.js b/ui/file_manager/file_manager/foreground/js/import_controller_unittest.m.js
index 0bbcb752ecb02..0394a90d16905 100644
--- a/ui/file_manager/file_manager/foreground/js/import_controller_unittest.m.js
+++ b/ui/file_manager/file_manager/foreground/js/import_controller_unittest.m.js
@@ -11,8 +11,8 @@ import {MockChromeStorageAPI, MockCommandLinePrivate} from '../../common/js/mock
 import {MockDirectoryEntry, MockFileEntry, MockFileSystem} from '../../common/js/mock_entry.m.js';
 import {reportPromise} from '../../common/js/test_error_reporting.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
-import {mediaImportInterfaces} from '../../externs/background/media_import_handler.m.js';
-import {mediaScannerInterfaces} from '../../externs/background/media_scanner.m.js';
+import {mediaImportInterfaces} from '../../externs/background/media_import_handler.js';
+import {mediaScannerInterfaces} from '../../externs/background/media_scanner.js';
 import {VolumeInfo} from '../../externs/volume_info.m.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 
diff --git a/ui/file_manager/file_manager/foreground/js/task_controller.js b/ui/file_manager/file_manager/foreground/js/task_controller.js
index 816d9246e9031..82be55dedd621 100644
--- a/ui/file_manager/file_manager/foreground/js/task_controller.js
+++ b/ui/file_manager/file_manager/foreground/js/task_controller.js
@@ -4,8 +4,8 @@
 
 // clang-format off
 // #import {FileTransferController} from './file_transfer_controller.m.js';
-// #import {ProgressCenter} from '../../externs/background/progress_center.m.js';
-// #import {Crostini} from '../../externs/background/crostini.m.js';
+// #import {ProgressCenter} from '../../externs/background/progress_center.js';
+// #import {Crostini} from '../../externs/background/crostini.js';
 // #import {NamingController} from './naming_controller.m.js';
 // #import {MetadataUpdateController} from './metadata_update_controller.m.js';
 // #import {DirectoryModel} from './directory_model.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/task_controller_unittest.m.js b/ui/file_manager/file_manager/foreground/js/task_controller_unittest.m.js
index 65371768760ae..1c91ce9bcb655 100644
--- a/ui/file_manager/file_manager/foreground/js/task_controller_unittest.m.js
+++ b/ui/file_manager/file_manager/foreground/js/task_controller_unittest.m.js
@@ -14,7 +14,7 @@ import {MockFileEntry, MockFileSystem} from '../../common/js/mock_entry.m.js';
 import {reportPromise} from '../../common/js/test_error_reporting.m.js';
 import {util} from '../../common/js/util.m.js';
 import {VolumeManagerCommon} from '../../common/js/volume_manager_types.m.js';
-import {ProgressCenter} from '../../externs/background/progress_center.m.js';
+import {ProgressCenter} from '../../externs/background/progress_center.js';
 import {VolumeManager} from '../../externs/volume_manager.m.js';
 
 import {DialogType} from './dialog_type.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/toolbar_controller.js b/ui/file_manager/file_manager/foreground/js/toolbar_controller.js
index a742a4b860db9..b891b1288b8ed 100644
--- a/ui/file_manager/file_manager/foreground/js/toolbar_controller.js
+++ b/ui/file_manager/file_manager/foreground/js/toolbar_controller.js
@@ -6,7 +6,7 @@
 // #import {FileListSelectionModel} from './ui/file_list_selection_model.js';
 // #import {A11yAnnounce} from './ui/a11y_announce.js';
 // #import {VolumeManager} from '../../externs/volume_manager.m.js';
-// #import {FileOperationManager} from '../../externs/background/file_operation_manager.m.js';
+// #import {FileOperationManager} from '../../externs/background/file_operation_manager.js';
 // #import {DirectoryModel} from './directory_model.m.js';
 // #import {LocationLine} from './ui/location_line.js';
 // #import {ListContainer} from './ui/list_container.js';
diff --git a/ui/file_manager/file_manager/foreground/js/ui/BUILD.gn b/ui/file_manager/file_manager/foreground/js/ui/BUILD.gn
index 4cb826a5f4e11..413a54b7cef9d 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/BUILD.gn
+++ b/ui/file_manager/file_manager/foreground/js/ui/BUILD.gn
@@ -221,7 +221,7 @@ js_library("directory_tree") {
     "//ui/file_manager/file_manager/externs:files_app_entry_interfaces.m",
     "//ui/file_manager/file_manager/externs:volume_info.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
     "//ui/file_manager/file_manager/foreground/js:constants.m",
     "//ui/file_manager/file_manager/foreground/js:directory_contents.m",
     "//ui/file_manager/file_manager/foreground/js:directory_model.m",
@@ -249,7 +249,7 @@ js_unittest("directory_tree_unittest.m") {
     "//ui/file_manager/file_manager/common/js:test_error_reporting.m",
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
-    "//ui/file_manager/file_manager/externs/background:file_operation_manager.m",
+    "//ui/file_manager/file_manager/externs/background:file_operation_manager",
     "//ui/file_manager/file_manager/foreground/js:directory_model.m",
     "//ui/file_manager/file_manager/foreground/js:fake_android_app_list_model.m",
     "//ui/file_manager/file_manager/foreground/js:mock_directory_model.m",
@@ -282,7 +282,7 @@ js_library("file_grid") {
     "//ui/file_manager/file_manager/common/js:util.m",
     "//ui/file_manager/file_manager/externs:files_app_entry_interfaces.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
+    "//ui/file_manager/file_manager/externs/background:import_history",
     "//ui/file_manager/file_manager/foreground/js:list_thumbnail_loader.m",
     "//ui/file_manager/file_manager/foreground/js/metadata:metadata_model",
     "//ui/webui/resources/js:assert.m",
@@ -399,7 +399,7 @@ js_library("file_table") {
     "//ui/file_manager/file_manager/externs:entry_location.m",
     "//ui/file_manager/file_manager/externs:files_app_entry_interfaces.m",
     "//ui/file_manager/file_manager/externs:volume_manager.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
+    "//ui/file_manager/file_manager/externs/background:import_history",
     "//ui/file_manager/file_manager/foreground/js:file_list_model.m",
     "//ui/file_manager/file_manager/foreground/js:list_thumbnail_loader.m",
     "//ui/file_manager/file_manager/foreground/js/metadata:metadata_model",
@@ -449,7 +449,7 @@ js_unittest("file_table_list_unittest.m") {
     "//ui/file_manager/file_manager/background/js:mock_volume_manager",
     "//ui/file_manager/file_manager/common/js:files_app_entry_types.m",
     "//ui/file_manager/file_manager/common/js:volume_manager_types.m",
-    "//ui/file_manager/file_manager/externs/background:import_history.m",
+    "//ui/file_manager/file_manager/externs/background:import_history",
     "//ui/file_manager/file_manager/foreground/js:directory_model.m",
     "//ui/file_manager/file_manager/foreground/js:file_list_model.m",
     "//ui/file_manager/file_manager/foreground/js/metadata:metadata_model",
diff --git a/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js b/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
index 72dd37d9ec7cc..6b48c607e7faa 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
@@ -13,7 +13,7 @@ import {FileType} from '../../../common/js/file_type.m.js';
 import {metrics} from '../../../common/js/metrics.m.js';
 import {str, util} from '../../../common/js/util.m.js';
 import {VolumeManagerCommon} from '../../../common/js/volume_manager_types.m.js';
-import {FileOperationManager} from '../../../externs/background/file_operation_manager.m.js';
+import {FileOperationManager} from '../../../externs/background/file_operation_manager.js';
 import {FilesAppDirEntry} from '../../../externs/files_app_entry_interfaces.m.js';
 import {VolumeInfo} from '../../../externs/volume_info.m.js';
 import {VolumeManager} from '../../../externs/volume_manager.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/ui/directory_tree_unittest.m.js b/ui/file_manager/file_manager/foreground/js/ui/directory_tree_unittest.m.js
index 47eac13cc8af3..2a3c524716da6 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/directory_tree_unittest.m.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/directory_tree_unittest.m.js
@@ -14,7 +14,7 @@ import {MockDirectoryEntry} from '../../../common/js/mock_entry.m.js';
 import {reportPromise, waitUntil} from '../../../common/js/test_error_reporting.m.js';
 import {str} from '../../../common/js/util.m.js';
 import {VolumeManagerCommon} from '../../../common/js/volume_manager_types.m.js';
-import {FileOperationManager} from '../../../externs/background/file_operation_manager.m.js';
+import {FileOperationManager} from '../../../externs/background/file_operation_manager.js';
 import {DirectoryModel} from '../directory_model.m.js';
 import {createFakeAndroidAppListModel} from '../fake_android_app_list_model.m.js';
 import {MetadataModel} from '../metadata/metadata_model.js';
diff --git a/ui/file_manager/file_manager/foreground/js/ui/file_grid.js b/ui/file_manager/file_manager/foreground/js/ui/file_grid.js
index 2d0da010678fb..b5d4bbcd0d2d9 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/file_grid.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/file_grid.js
@@ -14,7 +14,7 @@ import {AsyncUtil} from '../../../common/js/async_util.m.js';
 import {FileType} from '../../../common/js/file_type.m.js';
 import {importer} from '../../../common/js/importer_common.m.js';
 import {str, util} from '../../../common/js/util.m.js';
-import {importerHistoryInterfaces} from '../../../externs/background/import_history.m.js';
+import {importerHistoryInterfaces} from '../../../externs/background/import_history.js';
 import {FilesAppEntry} from '../../../externs/files_app_entry_interfaces.m.js';
 import {VolumeManager} from '../../../externs/volume_manager.m.js';
 import {ListThumbnailLoader} from '../list_thumbnail_loader.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/ui/file_table.js b/ui/file_manager/file_manager/foreground/js/ui/file_table.js
index 5898754ac865d..7dfcda90257d0 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/file_table.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/file_table.js
@@ -12,7 +12,7 @@ import {AsyncUtil} from '../../../common/js/async_util.m.js';
 import {FileType} from '../../../common/js/file_type.m.js';
 import {importer} from '../../../common/js/importer_common.m.js';
 import {str, strf, util} from '../../../common/js/util.m.js';
-import {importerHistoryInterfaces} from '../../../externs/background/import_history.m.js';
+import {importerHistoryInterfaces} from '../../../externs/background/import_history.js';
 import {EntryLocation} from '../../../externs/entry_location.m.js';
 import {FilesAppEntry} from '../../../externs/files_app_entry_interfaces.m.js';
 import {VolumeManager} from '../../../externs/volume_manager.m.js';
diff --git a/ui/file_manager/file_manager/foreground/js/ui/file_table_list_unittest.m.js b/ui/file_manager/file_manager/foreground/js/ui/file_table_list_unittest.m.js
index 5107e51488d4c..a1a63a2bebd69 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/file_table_list_unittest.m.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/file_table_list_unittest.m.js
@@ -7,7 +7,7 @@ import {assertEquals, assertFalse, assertTrue} from 'chrome://test/chai_assert.j
 import {MockVolumeManager} from '../../../background/js/mock_volume_manager.js';
 import {FakeEntryImpl} from '../../../common/js/files_app_entry_types.m.js';
 import {VolumeManagerCommon} from '../../../common/js/volume_manager_types.m.js';
-import {importerHistoryInterfaces} from '../../../externs/background/import_history.m.js';
+import {importerHistoryInterfaces} from '../../../externs/background/import_history.js';
 import {DirectoryModel} from '../directory_model.m.js';
 import {FileListModel} from '../file_list_model.m.js';
 import {MetadataModel} from '../metadata/metadata_model.js';
diff --git a/ui/file_manager/file_manager/manifest.json b/ui/file_manager/file_manager/manifest.json
index f0b74dfc72256..26cc08a002205 100644
--- a/ui/file_manager/file_manager/manifest.json
+++ b/ui/file_manager/file_manager/manifest.json
@@ -163,7 +163,7 @@
     "common/js/util.m.js",
     "common/js/volume_manager_types.m.js",
     "common/js/xfm.m.js",
-    "externs/background/background_base.m.js",
+    "externs/background/background_base.js",
     "externs/drive_dialog_controller.m.js",
     "externs/entry_location.m.js",
     "externs/files_app_entry_interfaces.m.js",
@@ -171,16 +171,16 @@
     "externs/volume_info.m.js",
     "externs/volume_info_list.m.js",
     "externs/volume_manager.m.js",
-    "externs/background/crostini.m.js",
-    "externs/background/drive_sync_handler.m.js",
-    "externs/background/duplicate_finder.m.js",
-    "externs/background/file_browser_background_full.m.js",
-    "externs/background/file_operation_manager.m.js",
-    "externs/background/import_history.m.js",
-    "externs/background/media_import_handler.m.js",
-    "externs/background/media_scanner.m.js",
-    "externs/background/progress_center.m.js",
-    "externs/background/task_queue.m.js",
+    "externs/background/crostini.js",
+    "externs/background/drive_sync_handler.js",
+    "externs/background/duplicate_finder.js",
+    "externs/background/file_browser_background_full.js",
+    "externs/background/file_operation_manager.js",
+    "externs/background/import_history.js",
+    "externs/background/media_import_handler.js",
+    "externs/background/media_scanner.js",
+    "externs/background/progress_center.js",
+    "externs/background/task_queue.js",
     "foreground/elements/files_icon_button.js",
     "foreground/elements/files_ripple.js",
     "foreground/elements/files_toast.js",
diff --git a/ui/file_manager/video_player/js/BUILD.gn b/ui/file_manager/video_player/js/BUILD.gn
index 9a07d6fce5b2f..8414a3d81fa76 100644
--- a/ui/file_manager/video_player/js/BUILD.gn
+++ b/ui/file_manager/video_player/js/BUILD.gn
@@ -58,7 +58,7 @@ js_library("background") {
     "//ui/file_manager/file_manager/background/js:app_window_wrapper",
     "//ui/file_manager/file_manager/background/js:background_base",
     "//ui/file_manager/file_manager/common/js:util.m",
-    "//ui/file_manager/file_manager/externs/background:background_base.m",
+    "//ui/file_manager/file_manager/externs/background:background_base",
     "//ui/webui/resources/js:assert.m",
   ]
 }
@@ -119,7 +119,7 @@ js_test_gen_html("js_test_gen_html_modules") {
                     "generate_exports=false",
                     "js_module_root=gen/ui/file_manager/",
                     "js_module_root=./gen/ui",
-                    "js_module_root=../../ui",
+                    "js_module_root=../../ui/file_manager/",
                     "browser_resolver_prefix_replacements=\"chrome://test/=./\"",
                     "browser_resolver_prefix_replacements=\"chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/=./file_manager/\"",
                     "browser_resolver_prefix_replacements=\"chrome-extension://pmfjbimdmchhbnneeidfognadeopoehp/=./image_loader/\"",
diff --git a/ui/file_manager/video_player/js/background.js b/ui/file_manager/video_player/js/background.js
index a71612ba7674b..d2c9c4927ebd9 100644
--- a/ui/file_manager/video_player/js/background.js
+++ b/ui/file_manager/video_player/js/background.js
@@ -5,7 +5,7 @@
 import {AppWindowWrapper} from 'chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/app_window_wrapper.js';
 import {BackgroundBaseImpl} from 'chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_base.js';
 import {util} from 'chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/common/js/util.m.js';
-import {BackgroundBase} from 'chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/externs/background/background_base.m.js';
+import {BackgroundBase} from 'chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/externs/background/background_base.js';
 import {assert} from 'chrome://resources/js/assert.m.js';
 
 /**