code: def file_serializer(obj):
      
      return {
          "id": str(obj.file_id),
          "filename": obj.key,
          "filesize": obj.file.size,
          "checksum": obj.file.checksum,
      }
docs: Serialize a object.
  
      :param obj: A :class:`invenio_files_rest.models.ObjectVersion` instance.
      :returns: A dictionary with the fields to serialize.