default_filter
Description:
The default filter, that is, the filter that is initially active in the file chooser dialog.
If the default filter is null, the first filter of [property@Gtk.FileDialog:filters] is used as the default filter. If that property contains no filter, the dialog will be unfiltered.
If [property@Gtk.FileDialog:filters] is not null, the default filter should be part of the list. If it is not, the dialog may choose to not make it available.