dont reupload file(s) when selecting existing ones

fixes bug where selecting an existing file in the UI would re-trigger
the last file(s) upload process

closes CNVS-15678

test plan:
  - enable new files
  - upload a file
  - select any file in the UI
    - a new upload of the previously upload file(s) should not happen

Change-Id: I572855a1cfb9d77d40d84960aa98c4fc9e951e75
Reviewed-on: https://gerrit.instructure.com/41533
Reviewed-by: Sterling Cobb <sterling@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>