In the Filesystem section for that app in Flatseal, you need to add the path to your NAS drive (the same SMB path that it's mounted in the Files app). That will give your FF flatpak access to that location.

In the Filesystem section for that app in Flatseal, you need to add the path to your NAS drive (the same SMB path that it's mounted in the Files app). That will give your FF flatpak access to that location.
