Add leading dot to our extension for `showSaveFilePicker` calls.

This change is required since the API was updated in http://crrev/c/2360597.

Bug: b/165769795
Change-Id: I918d5bfa0ab7887e356f91465425d28f6b73c1bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366380
Commit-Queue: Mitchell McDermott <mcdermottm@google.com>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800050}

chromeos/components/media_app_ui/resources/js/launch.js

@@ -290,7 +290,7 @@ guestMessagePipe.registerHandler(Message.OPEN_FILE, async () => {
  * @return {!Promise<!FileSystemFileHandle>}
  */
 function pickWritableFile(suggestedName, mimeType) {
-  const extension = suggestedName.split('.').reverse()[0];
+  const extension = '.' + suggestedName.split('.').reverse()[0];
   // TODO(b/161087799): Add a default filename when it's supported by the
   // native file api.
   /** @type {!FilePickerOptions} */

chromeos/components/media_app_ui/test/media_app_ui_browsertest.js

@@ -921,8 +921,8 @@ TEST_F('MediaAppUIBrowserTest', 'RequestSaveFileIPC', async () => {
   const lastToken = [...tokenMap.keys()].slice(-1)[0];
   assertMatch(result.testQueryResult, lastToken);
   assertEquals(options.types.length, 1);
-  assertEquals(options.types[0].description, 'png');
-  assertDeepEquals(options.types[0].accept['image/png'], ['png']);
+  assertEquals(options.types[0].description, '.png');
+  assertDeepEquals(options.types[0].accept['image/png'], ['.png']);
   testDone();
 });