theme editor: only allow png apple-touch-icons
theme editor: only allow png apple-touch-icons

fixes: CNVS-22177

test plan:
* go to the theme editor and try to upload a
 "Mobile Homescreen Icon"
* the helper text should tell you can only use pngs
* and the file picker should only be able to pick
  png files

Change-Id: I9ef7402bd08fdb70512604eefce5e0da1ae8abbb
Reviewed-on: https://gerrit.instructure.com/59406
Tested-by: Jenkins
Reviewed-by: Jennifer Stern <7e594e5cba47371ac9ee694a84b9d94f9f6ec815@instructure.com>
Product-Review: Jennifer Stern <7e594e5cba47371ac9ee694a84b9d94f9f6ec815@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>

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