public function validateFileType() {
        // if the allowed types is null, all file types are allowed
        if (is_null($this->allowedTypes())) {
            return true;
        }
        
        // if the download file type is not in the list with allowed file types,
        // or if the allowed mime types list is not an array, throw a new exception
        if (!is_array($this->allowedTypes()) || !in_array($this->mimeType(), $this->allowedTypes())) {
            throw new \Th\FileDownloader\Exceptions\DownloadException(\Th\FileDownloader::FILE_WRONG_MIME);
        }
        
        // everything ok
        return true;
    }