public function validate_url()
 {
     $file_url = $_REQUEST['file_url'];
     $info = \Podlove\Model\MediaFile::curl_get_header_for_url($file_url);
     $header = $info['header'];
     $reachable = $header['http_code'] >= 200 && $header['http_code'] < 300;
     $validation_cache = get_option('podlove_migration_validation_cache', array());
     $validation_cache[$file_url] = $reachable;
     update_option('podlove_migration_validation_cache', $validation_cache);
     self::respond_with_json(array('file_url' => $file_url, 'reachable' => $reachable, 'file_size' => $header['download_content_length']));
 }