The Gtk.FileFilter implementation of the Gtk.Buildable interface supports adding rules using the , and elements and listing the rules within. Specifying a or has the same effect as as calling Gtk.FileFilter.add_mime_type () or Gtk.FileFilter.add_pattern ().
Thanks. Tricky.. gnome libraries currently use either markdown or docbook and they don't specify which one they use. In this case the tags get interpreted as XML as they are not escaped.
For example, in Gtk.FileFilter:
They should say