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