private static function makeHashFilename($key, $make_group_directory = false)
    {
        $hash = hash('sha256', '/bow_'.$key);

        $group = Str::slice($hash, 0, 2);

        if ($make_group_directory) {
            if (!is_dir(static::$directory.'/'.$group)) {
                @mkdir(static::$directory.'/'.$group);
            }
        }

        return static::$directory.'/'.$group.'/'.$hash;
    }