public function setCrossDomainPolicy($policy = self::ALL)
    {
        $this->checkValues($policy, [
            self::ALL,
            self::NONE,
            self::MASTER_ONLY,
            self::BY_CONTENT_TYPE,
            self::BY_FTP_FILENAME,
        ]);
        $this->headers['x-permitted-cross-domain-policies'] = $policy;

        return $this;
    }