docs: def delete_attachments(self, volumeID, attachmentsID):
          ''' delete attachments from a volume '''
          log.debug("deleting attachments from volume '{}': {}".format(volumeID, attachmentsID))
          rawVolume = self._req_raw_volume(volumeID)
          insID = [a['id'] for a in rawVolume['_source']['_attachments']]
          # check that all requested file are present
          for id in attachmentsID:
              if id not in insID:
                  raise NotFoundException("could not found attachment '{}' of the volume '{}'".format(id, volumeID))
          for index, id in enumerate(attachmentsID):
              rawVolume['_source']['_attachments'].pop(insID.index(id))
          self._db.modify_book(volumeID, rawVolume['_source'], version=rawVolume['_version'])