saveToDownloads method
Implementation
@override
Future<String> saveToDownloads(String path, String? fileName) async {
await _ensureImageLoaded(); // only needs the engine's download() helper
// Never default to .mp4 — this method is shared with the image API, and a
// JPEG saved as "compressed.mp4" is worse than a generic name. The JS side
// derives the extension from the blob's MIME type when we pass null.
return (await _jsDownload(path, fileName).toDart).toDart;
}