public function setDescription($description)
    {
        if (is_null($description)) {
            unset($this->_headers['content-description']);
        } else {
            if (!($hdr = $this->_headers['content-description'])) {
                $hdr = new Horde_Mime_Headers_ContentDescription(null, '');
                $this->_headers->addHeaderOb($hdr);
            }
            $hdr->setValue($description);
        }
    }