code: public static MimeBodyPart newHtmlAttachmentBodyPart( URL contentUrl, String contentId )
      throws MessagingException
    {
      MimeBodyPart mimeBodyPart = new MimeBodyPart();
      mimeBodyPart.setDataHandler(new DataHandler(contentUrl));
      if( contentId != null ) {
        mimeBodyPart.setHeader("Content-ID", contentId);
      }
      return mimeBodyPart;
    }
docs: Creates a body part for an attachment that is used by an html body part.