docs: def files(self):
          """Get files iterator.
  
          :returns: Files iterator.
          """
          if self.model is None:
              raise MissingModelError()
  
          records_buckets = RecordsBuckets.query.filter_by(
              record_id=self.id).first()
  
          if not records_buckets:
              bucket = self._create_bucket()
              if not bucket:
                  return None
              RecordsBuckets.create(record=self.model, bucket=bucket)
          else:
              bucket = records_buckets.bucket
  
          return self.files_iter_cls(self, bucket=bucket, file_cls=self.file_cls)