code: def getAttachment(self, oid, attachment_id, out_folder=None):
          
          attachments = self.listAttachments(oid=oid)
          if "attachmentInfos" in attachments:
              for attachment in attachments['attachmentInfos']:
                  if "id" in attachment and \
                     attachment['id'] == attachment_id:
                      url = self._url + "/%s/attachments/%s" % (oid, attachment_id)
                      return self._get(url=url, param_dict={"f":'json'},
                                       securityHandler=self._securityHandler,
                                       out_folder=out_folder,
                                       file_name=attachment['name'])
          return None
docs: downloads a feature's attachment.
  
          Inputs:
             oid - object id of the feature
             attachment_id - ID of the attachment.  Should be an integer.
             out_folder - save path of the file
          Output:
             string - full path of the file