I have tried a few options, none of which seem to work (if I have a simple multipart form with a named field, it works well, but when I don't know the name I can't just grab all files in the request...).
I have looked at http://stackoverflow.com/questions/81451/upload-files-in-google-app-engine and it doesn't seem suitable (or to actually work, as someone mentioned the code snipped it untested).