Make Digikam the default photo application on Linux Mint via Flatpak

[Damien]

Hello there,
I'm new to both Linux and Digikam. I recently started on Linux Mint (19, now 20) and installed Digikam 7.0 via Flatpak. The application seems to work fine, but I cannot make it the default app for pictures.
I found where this setting is managed, but Digikam does not appear in the app list, not even in "other applications".

Am I doing something wrong?
Damien