Comment 15 for bug 1958670

I've pushed some more changes to https://github.com/PintaProject/Pinta/tree/feature/native-save-dialogs
It now adds an extension to the initial placeholder filename when saving a new file, and also just uses the native save dialog as-is (using the selected filename without any modifications, which should fix the portal issue)

The downside is that the GTK file dialog doesn't auto-update the extension if you change the filter (https://gitlab.gnome.org/GNOME/gtk/-/issues/402), and also doesn't provide a signal to let us do that ourselves (https://gitlab.gnome.org/GNOME/gtk/-/issues/4626). But this would be the standard behaviour that all other GTK apps on the system would have so it's not an odd behaviour specific to Pinta...