webp files not displaying in Backgrounds thumbnails, nor on desktop

Distribution

21.3 (Virginia)

Package version

6.0.4

Graphics hardware in use

Radeon RX 480

Frequency

Always

Bug description

I've got several webp files in a directory that I have added to Settings->Backgrounds and chosen as the directory for desktop wallpaper. None of them show up in the grid of thumbnails that's part of the Backgrounds and none of them are displayed as desktop wallpaper.

Steps to reproduce

Create a webp file. Put it in a directory. Add that directory to Backgrounds. The file will not appear as a thumbnail, nor will it be rendered on the desktop.

Expected behavior

Given that I can set the desktop manually with gsettings set org.cinnamon.desktop.background picture-uri 'file:///<path to webp file>, I'd expect Cinnamon Backgrounds to read this file format and display its thumbnail and set it as a desktop.

Additional information

This seems related to #11746 and fe682482e781f01bae1b51f22c52892e0ea26913.

Aside from waiting for that commit to make it out of the 5.8-maintenance branch, how might I workaround this? Is there a way of downgrading the pillow dependency? If I apply the patch to the files in situ on the filesystem, would that be enough, or would I need to delete cached pyc objects?

Start by reading related issue #11746 and commit fe682482e781f01bae1b51f22c52892e0ea26913, then trace Cinnamon Backgrounds handling for the reported webp files and its Pillow dependency. Done means webp files appear as Backgrounds thumbnails and can be rendered as desktop wallpaper.