public function addPart($mime_type, $content, $charset = 'us-ascii',
                            $disposition = null)
    {
        $part = new Horde_Mime_Part();
        $part->setType($mime_type);
        $part->setCharset($charset);
        $part->setDisposition($disposition);
        $part->setContents($content);
        return $this->addMimePart($part);
    }