Python mechanize：提交带有文件（图像）的表单？

有一个表单具有以下控件（它是一个上载图像控件）

我试过了

   br.form['profile_image'] = open("img.jpg")

但是，我得到了错误

  File "/usr/local/lib/python2.6/dist-packages/mechanize-0.2.4-py2.6.egg/mechanize/_form.py", line 2784, in __setitem__
    raise ValueError(str(e))
ValueError: value attribute is readonly

这样做：

br.form.add_file(open(FILENAME), 'text/plain', FILENAME)
br.form.set_all_readonly(False)

  File "/usr/local/lib/python2.6/dist-packages/mechanize-0.2.4-py2.6.egg/mechanize/_form.py", line 2784, in __setitem__
    raise ValueError(str(e))
ValueError: value attribute is readonly

br.form.add_file(open(FILENAME), 'text/plain', FILENAME)
br.form.set_all_readonly(False)