Seek back to the beginning when uploading a file (bug 1045764)

https://bugzilla.mozilla.org/show_bug.cgi?id=1045764

Some of our code might read the beginning of the file returned by the form before handing it over to `FileUpload.from_post()`. This is because the API emulates a regular file upload process but directly creates a `StringIO` object. Every packaged app upload using the API was invalid as a result.
drive-by optimisation, avoid a second `save()`.