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 gtk.file_dialog.FileDialog.filters is used as the default filter. If that property contains no filter, the dialog will be unfiltered.
If gtk.file_dialog.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.
Set defaultFilter property.