def updateAttachment(self, oid, attachment_id, file_path):
        """ updates an existing attachment with a new file
            Inputs:
               oid - string/integer - Unique record ID
               attachment_id - integer - Unique attachment identifier
               file_path - string - path to new attachment
            Output:
               JSON response
        """
        url = self._url + "/%s/updateAttachment" % oid
        params = {
            "f":"json",
            "attachmentId" : "%s" % attachment_id
        }
        files = {'attachment': file_path }
        res = self._post(url=url,
                             param_dict=params,
                             files=files,
                             securityHandler=self._securityHandler,
                             proxy_port=self._proxy_port,
                             proxy_url=self._proxy_url)
        return self._unicode_convert(res)
updates an existing attachment with a new file
            Inputs:
               oid - string/integer - Unique record ID
               attachment_id - integer - Unique attachment identifier
               file_path - string - path to new attachment
            Output:
               JSON response