def test_native(self):
    self.app.redirect_to("/pagesMall/release/index")
    self.page.get_element("view.upload").click()
    image_name = "test2.png" 
    with open(image_name, "rb") as fd:
        c = fd.read()
        image_b64data=base64.b64encode(c).decode("utf-8")
    self.app.mock_choose_image(image_name, image_b64data)
b部分代码片段，参考的微信官方文档