docs: def insert_attachments(self, volumeID, attachments):
          ''' add attachments to an already existing volume '''
          log.debug("adding new attachments to volume '{}': {}".format(volumeID, attachments))
          if not attachments:
              return
          rawVolume = self._req_raw_volume(volumeID)
          attsID = list()
          for index, a in enumerate(attachments):
              try:
                  rawAttachment = self._assemble_attachment(a['file'], a)
                  rawVolume['_source']['_attachments'].append(rawAttachment)
                  attsID.append(rawAttachment['id'])
              except Exception:
                  log.exception("Error while elaborating attachments array at index: {}".format(index))
                  raise
          self._db.modify_book(volumeID, rawVolume['_source'], version=rawVolume['_version'])
          return attsID