def METHOD_NAME(self, mock_open):
    view = View(Item("filepath", editor=FileEditor(truncate_ext=True)))
    obj = FileModel()
    tester = UITester()
    with tester.create_ui(obj, dict(view=view)) as ui:
        editor = ui.get_editors("filepath")[0]
        editor.show_file_dialog()
        self.assertEqual(editor.value, "some_file")