code: 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.');
          }
      }
docs: Write Default content type.
  
  @param XMLWriter $objWriter XML Writer
  @param string $pPartname Part name
  @param string $pContentType Content type
  
  @throws WriterException