And without reading the bug, the issue with adding thumbnail support to that one is probably because thumbnails need to be cached to be fast, and it would be odd for a filepicker to be creating its own cache files (since it can't make any assumptions about the system it runs on).
It's not that. Firstly there is a standard for where to cache thumbnails at least on Linux, and secondly the file picker already generates (or at least displays) thumbnails. It's shown in the article. It's just that GTK has no suitable widget for showing them properly.
