How to use the @jupyterlab/filebrowser.FileButtons function in @jupyterlab/filebrowser

openedWidget = null;
    commands = new CommandRegistry();
    registry = new DocumentRegistry();
    registry.addModelFactory(modelFactory);
    registry.addWidgetFactory(widgetFactory);
    manager = new DocumentManager({
      registry,
      manager: services,
      opener: {
        open: (widget: Widget) => {
          openedWidget = widget;
        }
      }
    });
    model = new FileBrowserModel({ manager: services });
    buttons = new FileButtons({ model, manager, commands });
  });