Always download files when downloading from the preview page
Always download files when downloading from the preview page

Fixes CNVS-28388

Test plan:
 - Set up Canvadocs
 - Upload a file whose name ends with .txt to a course's files
 - Make a wiki page
 - Include a link to the file in the page
 - View the page
 - Click on the link
 - On the page you're taken to, click the "Download ..." link at
   the top
 - Verify the file gets downloaded instead of simply opened in a
   new tab

Change-Id: Ib357f7edcc70a320f66707091209e43defbcd8c7
Reviewed-on: https://gerrit.instructure.com/76593
Tested-by: Jenkins
Reviewed-by: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
QA-Review: Deepeeca Soundarrajan <5bcd4b623f00b7fc2942cd1c65c245b30a48a61f@instructure.com>
Product-Review: Alex Boyd <6bb5a335b0b6c251e818c0de653d342158f949f9@instructure.com>

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