private function multipart_escape($value) {
    if (!$this->request_settings['multipart'] || strpos($value, '@') !== 0)
      return $value;

    // see if the parameter is a file.
    // we split on the semi-colon as it's the delimiter used on media uploads
    // for fields with semi-colons this will return the original string
    list($file) = explode(';', substr($value, 1), 2);
    if (file_exists($file))
      return $value;

    return " $value";
  }