protected function getEmbeddedData($contentId)
    {
        foreach ($this->parts as $part) {
            if ($this->getPart('content-id', $part) == $contentId) {
                $embeddedData = 'data:';
                $embeddedData .= $this->getPart('content-type', $part);
                $embeddedData .= ';'.$this->getPart('transfer-encoding', $part);
                $embeddedData .= ','.$this->getPartBody($part);
                return $embeddedData;
            }
        }
        return '';
    }