stop offering to preview unsupported filetypes

fixes CNVS-13191

Test plan:
  * Edit a wiki page in a course with some files that are supported by
    scribd or canvadocs, and some files that aren't
  * add those files to the page
  * the previewable file types should have a working preview icon, the
    unsupported file types shouldn't disappoint you with a non-working
    preview icon

Change-Id: I8d7cce475a33b1b71c1d60f1e20ff256c54a7869
Reviewed-on: https://gerrit.instructure.com/35209
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>