picFile method
Opens the system file picker to select a document.
Returns the selected file's path if successful, otherwise null.
Implementation
Future<File?> picFile() {
  assert(
      !kIsWeb &&
          (mediaType == MediaType.document || mediaType == MediaType.audio),
      'Only audio and documents are supported as file');
  return PickerProMaxUltra().picFile(mediaType: mediaType);
}