public function knownExtensions($type = null)
    {
        $extensions = [
            'images'   => static::$imageExtensions,
            'audios'    => static::$audiosExtensions,
            'archives' => static::$archivesExtensions,
            'videos'   => static::$videosExtensions,
        ];

        if ($type) {
            if (in_array($type, array_keys($extensions))) {
                return $extensions[$type];
            }

            return null;
        }

        return $extensions;
    }