private function renderEmbeddedFiles(&$content, $boundary)
    {
        foreach ($this->cids as $image => $cid) {
            if (!is_readable($image)) {
                throw new MailException('Image could bot be read: ' . $image);  // @codeCoverageIgnore
            }
            $type = self::getMimeTypeOfImage($image);
            $name = basename($image);
            $data = chunk_split(base64_encode(file_get_contents($image)));
            $content .= "\r\n";
            $content .= "--$boundary\r\n";
            $content .= "Content-Type: $type;\r\n\tname=\"$name\"\r\n";
            $content .= "Content-Transfer-Encoding: base64\r\n";
            $content .= "Content-ID: <" . $cid . ">\r\n";
            $content .= "Content-Description: \"$name\"\r\n";
            $content .= "Content-Location: \"$name\"\r\n";
            $content .= "\r\n";
            $content .= "$data";
        }
    }