support uploading file content to addons submission api

[eviljeff]

https://addons-server.readthedocs.io/en/latest/topics/api/addons.html supports some fields which are set or updated by uploading a file, as multipart-formdata. The addon icon, version source, and preview images are all file uploads - they are all also sent as subsequent PATCH requests* because JSON and form data encoding can't be combined.

*a general update/edit function for addons/versions (i.e. without uploading a new xpi) would also be useful, but is a slightly different ask, so would be a separate issue (it would however share a lot of code).