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

Some of our code might read the beginning of the file returned by
the form before handing it over to FileUpload.from_post().
seek might not be present, chunks can be a list in some cases
Merge pull request #2403 from diox/fix-api-packaged-apps

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