_gtk_file_filter_get_as_pattern_nsstrings() should match GLib's macOS implementation of g_content_type_from_mime_type()
The logic in GTK does not do enough work to convert a mime type to a UTI. This makes filters broken with the GtkFileChooserNative API on macOS.
However GLib already handles this: https://gitlab.gnome.org/GNOME/glib/-/blob/main/gio/gosxcontenttype.m#L372
So GTK should probably just call the GContentType APIs.
Edited by Patrick Griffis