Thumbnail rendering on GNOME is already sandboxed with bubblewrap, even when you're outside Flatpak: [0] for docs and [1] for code on how they accomplish it.

I don't think it's possible yet to ship thumbnailers in Flatpak - I'll try poking someone to see if there are plans for that.

[0]: https://gitlab.gnome.org/GNOME/gnome-desktop/-/blob/master/R... [1]: https://gitlab.gnome.org/GNOME/gnome-desktop/-/blob/master/l...