Do not apply transparent image background pattern for thumbnails in Phriction
Summary: Closes T16415
Test Plan:
* Create a transparent image file.
* Create a Phriction document which embeds that file - see no checkerbox pattern anymore and no pattern when hovering either
* Create a Phame document which embeds that file - see no checkerbox pattern anymore and no pattern when hovering either
* Create a Maniphest task which embeds that file - still see that checkerbox pattern and still see a change when hovering
* Go to its File page - still see that checkerbox pattern and still see a change when hovering
* Create a Pholio mockup using that file - still see that checkerbox pattern and still see no change when hovering
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16415
Differential Revision: https://we.phorge.it/D26628