public function _updateContentType()
    {
        unset($this->_headers['Content-Type']);
        $suffix = '';
        if ($this->isMultipart()) {
            $suffix = '; boundary=' . $this->boundary();
        } elseif ($this->_charset) {
            $suffix = '; charset=' . $this->_charset . $suffix;
        }
        if ($this->_mime) {
            $this->_headers['Content-Type'] = $this->_mime . $suffix;
        }
    }