Makes file preview work for user files

fixes CNVS-17683

Test Plan:
	- Enable New Files
	- Go to the user's files (/files)
	- Navigate to a course's files within the user's files
	- Preview a file, it should preview without a problem
	- Go to one of the users personal files
	- It should also preview just fine.

Change-Id: I7e688f25bef06a5dc8e5f66ff2ee6535d7b50ba6
Reviewed-on: https://gerrit.instructure.com/46361
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>