Deletes files modified more than a set time ago.
Callback for file_scan_directory() within:
function drupal_delete_file_if_stale($uri) {
// Default stale file threshold is 30 days.
if (REQUEST_TIME - filemtime($uri) > config('system.performance')
->get('stale_file_threshold')) {
file_unmanaged_delete($uri);
}
}