public function view($id, $format = null)
 {
     $file = File::findOrFail($id);
     $pathname = $file->getPathname($format);
     if (!file_exists($pathname)) {
         $file->makeSmallerSize($format);
     }
     if (!file_exists($pathname)) {
         abort(404);
     }
     return response()->download($pathname, $file->name, [], 'inline');
 }