code: public static MimeBodyPart newAttachmentBodyPart( URL contentUrl, String contentId )
      throws MessagingException
    {
      MimeBodyPart mimeBodyPart = new MimeBodyPart();
      mimeBodyPart.setDataHandler(new DataHandler(contentUrl));
      if( contentId != null ) {
        mimeBodyPart.setHeader("Content-ID", contentId);
      }
      mimeBodyPart.setDisposition(DISPOSITION_ATTACHMENT);
      String fileName = fileNameForUrl(contentUrl);
      if( fileName != null ) {
        mimeBodyPart.setFileName(fileName);
      }
      return mimeBodyPart;
    }
docs: Creates a body part for an attachment that is downloaded by the user.