public function syncContentType()
    {
        $stream = reset($this->_streams);
        if (!$this->isMultipart() && $stream) {
            unset($this->_headers['Content-Type']);
            $this->mime($stream->mime());
            $this->charset($stream->charset());
            unset($this->_headers['Content-Transfer-Encoding']);
            if ($encoding = $stream->encoding()) {
                $this->_headers['Content-Transfer-Encoding'] = $encoding;
            }
        }
    }