private function writeDefaultContentType(XMLWriter $objWriter, $pPartname, $pContentType)
    {
        if ($pPartname != '' && $pContentType != '') {
            // Write content type
            $objWriter->startElement('Default');
            $objWriter->writeAttribute('Extension', $pPartname);
            $objWriter->writeAttribute('ContentType', $pContentType);
            $objWriter->endElement();
        } else {
            throw new WriterException('Invalid parameters passed.');
        }
    }