fix /files/preflight route

test plan: you should be able to upload files to the files page
with Better File Browsing disabled, and Flash enabled in the
browser.

fixes CNVS-16126

Change-Id: Ieafb0030d9228eee846ff0cf326dcf83ceb5b8b2
Reviewed-on: https://gerrit.instructure.com/42347
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>