private function generateNewFilename()
    {
        $time = time();
        $filename = $this->filename;
        $ext =  end(explode(".", $filename));
        do {
            $filename = substr(md5($filename. $time), 0, 8) . '.' . $ext;
        } while (file_exists($this->uploadDirectory . DIRECTORY_SEPARATOR . $filename));
        return $filename;
    }