public static function getMimeTypeFromFilename($fileName)
    {
        $ext = strtolower(pathinfo($fileName, PATHINFO_EXTENSION));
        if (array_key_exists($ext, self::$customMimeTypes)) {
            return self::$customMimeTypes[$ext];
        } else if (array_key_exists($ext, self::$mimeTypes)) {
            return self::$mimeTypes[$ext];
        }
        return 'application/octet-stream';
    }