code: private Part createTempPart(Attachment a) {
          return Part.builder().setName(String.valueOf(a.hashCode())).setSize(0).setType(Attachment.LOCAL_PART_TYPE_UPLOADING).setUrl(null).setData(null).build();
      }
docs: Create message part based on attachment upload. This is a temporary message to indicate that one of the attachments for this message is being uploaded.
  
  @return Message part.