add attachments to an already existing volume

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