Above is Files 3.12.1 and we have the classical folders icons. Let’s make things more fancy and display richer thumbnails that previewing the directories content similar to what Photos album do, or Shell Applications Folders.
Zoom it a bit..
This could actually be a superb idea if it had a more proper implementation, like options from within Files-App.
By the way, you can delete generated thumbnails from cache
$ rm -rf ~/.cache/thumbnails/* #don't CnP such commands!
The previous two figures are from Files 3.13.x and you would noticed a small visual change. On Places-Sidebar, Files is now using separators (horizontal lines) instead of labels.