docs: def init_app(self, app, entry_point_group='invenio_previewer.previewers'):
          """Flask application initialization."""
          self.init_config(app)
          app.register_blueprint(blueprint)
          state = _InvenioPreviewerState(
              app,
              entry_point_group=entry_point_group)
          app.extensions['invenio-previewer'] = state
          return state