docs: def denormalize_attachment(attachment):
          '''convert attachment metadata from archivant to es format'''
          res = dict()
          ext = ['id', 'url']
          for k in ext:
              if k in attachment['metadata']:
                  raise ValueError("metadata section could not contain special key '{}'".format(k))
              res[k] = attachment[k]
          res.update(attachment['metadata'])
          return res