Font-size change in _FilePreview.scss
Font-size change in _FilePreview.scss

refs: CNVS-25278

Test Plan:
- This is a 1 to 1 change. Font size will just be relative instead of
fixed sized (ie, rem instead of px)
- CSS should compile successfully
- In new files, preview an image. Arrows for prev/next should look the
same

Change-Id: Iea3371c72c7c1e24754417f9cf176a983a690cc8
Reviewed-on: https://gerrit.instructure.com/72489
Tested-by: Jenkins
Reviewed-by: Pam Hiett <7b533437add1ce4ea4ed79979fa88e0e484e554b@instructure.com>
Product-Review: Pam Hiett <7b533437add1ce4ea4ed79979fa88e0e484e554b@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>

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