message: move callback to match position with source
diff: @@ -25,6 +25,11 @@ declare class FilePicker<P extends FilePicker.Options = FilePicker.Options> exte
  */
  activeSource: string;
  + /**
  + * A callback function to trigger once a file has been selected
  + */
  + callback: FilePicker.Callback | undefined;
  +
  /**
  * The latest set of results browsed from the server
  * @remarks This is never set.
  @@ -56,11 +61,6 @@ declare class FilePicker<P extends FilePicker.Options = FilePicker.Options> exte
  */
  extensions: string[];
  - /**
  - * A callback function to trigger once a file has been selected
  - */
  - callback: FilePicker.Callback | undefined;
  -
  protected _loaded: boolean;
  /**
  