unbreak legacy JSON files endpoint with new files enabled

test plan:
 - enable new files in a course
 - create a wiki page in the course
 - click "upload a new file" in the wiki sidebar
 - there should not be an ajax error, and the select box
   to choose the folder to uplod to should populate

fixes CNVS-17359

Change-Id: I1a996eecd10e101d27c391da5f1b7be65296bb5d
Reviewed-on: https://gerrit.instructure.com/45485
Reviewed-by: Dan Minkevitch <dan@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>