It was inside of a global form indeed.
I couldn't do the upload though, after a couple days worth of troubleshooting.
However I finally managed to encode the raw http response properly by saving the request sent in py2 to a file and carefully checking py3 one against it.
I'll save mechanize for some other time
Thanks again for your help with this!
I'll take discussion of the plugin itself to an appropriate branch, once I tested it more, but for now it seems to work with py3, so yay.