How to make Thunderbird use a Flatpak app as default file handler?

I'm trying to make Flatpak Libreoffice the default app for opening files in Thunderbird.

At the moment, Thunderbird (also a flatpak) is using System Handler as default. I tried changing it but cannot find where the libreoffice executable is on my system.

I found the libreoffice .desktop files in /var/lib/flatpak/exports/share/applications but Thunderbird doesn't seem to recognise the app?