Unfortunately the answer is "Yes".

TestApp.encode_multipart has this bit:

            if isinstance(value, forms.File):
                if value.value:
                    _append_file([key] + list(value.value))

Turns out the `value` attribute of `forms.File` is an instance of `forms.Upload`, so the content type gets lost here without that extra yield. :-(

I wish I'd discovered this before asking for the 2.0.8 release to PyPI.