code: public static UploadAttachmentResponse uploadAttachment(
  			AttachmentType attachmentType, String attachmentUrl) {
  		AttachmentPayload payload = new AttachmentPayload(attachmentUrl, true);
  		Attachment attachment = new Attachment(attachmentType, payload);
  		AttachmentMessage message = new AttachmentMessage(attachment);
  		FbBotMillMessageResponse toSend = new FbBotMillMessageResponse(null, message);
  		return FbBotMillNetworkController.postUploadAttachment(toSend);
  	}
docs: Method to upload an attachment to Facebook's server in order to use it
  later. Requires the pages_messaging permission.
  
  @param attachmentType
  the type of attachment to upload to Facebook. Please notice
  that currently Facebook supports only image, audio, video and
  file attachments.
  @param attachmentUrl
  the URL of the attachment to upload to Facebook.
  @return nonexpiring ID for the attachment.