docs: def get_version(self, version_id=None):
          """Return specific version ``ObjectVersion`` instance or HEAD.
  
          :param version_id: Version ID of the object.
          :returns: :class:`~invenio_files_rest.models.ObjectVersion` instance or
              HEAD of the stored object.
          """
          return ObjectVersion.get(bucket=self.obj.bucket, key=self.obj.key,
                                   version_id=version_id)