fix doc previews for files uploaded via the wiki sidebar

test plan:
 In an environment with canvadocs enabled,
 * upload a previewable document type to a wiki page via the
   wiki sidebar
 * save the page
 * the document preview icon should appear next to the link
   and clicking it should open a document preview

fixes CNVS-15939

Change-Id: I9a52c01bc128760885114e71271bc1c477853a56
Reviewed-on: https://gerrit.instructure.com/45377
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>