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;
}