Override the behavior of title_query(). Get the filenames.
Overrides Numeric::title_query
function title_query() {
  $titles = db_select('file_managed', 'f')
    ->fields('f', array(
    'filename',
  ))
    ->condition('fid', $this->value)
    ->execute()
    ->fetchCol();
  foreach ($titles as &$title) {
    $title = check_plain($title);
  }
  return $titles;
}