UploadBase: Stop mLocalFile doubling as stashed file

UploadBase: Stop mLocalFile doubling as stashed file

"I've a great idea", they said. "You know what would be cool? If I
made this boring getter, getLocalFile(), return something completely
different after the file was stashed. This will be a nice surprise for
someone in the future to discover", they added gleefully.

I am pretty sure everything still works, but I never could get async
upload publishing to work locally, so I'd appreciate some testing.

Change-Id: I11dcf2ed89e4f1dd8ddf081af521da005efdbf39