backend: Only build once the list of upload files

The list of upload files is built currently at every UploadFile() call.
This patch moves it to a separate variable which is initialized only
once.

This won't make much difference but I regard it as cleanup.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>