Thumbnails for folders

We could show thumbnails for folders instead of a generic icon to make it easily identifiable for the user.
The thumbnail could be autogenerated based on the content inside the folder or could even be customizable so that the user can choose a thumbnail of their liking.

The issue describes an Android app feature but names no files, tests, or entry points. Start by locating the folder rendering in the file-browser UI and clarify whether thumbnails are autogenerated, customizable, or both. Done requires an agreed thumbnail behavior and visible folder thumbnails that replace the generic icon.

Written by the indexing model from the issue text.