public static function getSubTypeList($type) {
        if (empty(self::$_subTypes[$type])) {
            throw new InvalidArgumentException(sprintf('Sub-type %s does not exist', $type));
        }

        $types = array();

        foreach (self::$_subTypes[$type] as $ext) {
            $types[$ext] = self::getTypeFromExt($ext);
        }

        return $types;
    }