macos: Update docs on UTI mapping

docs/reference/gtk/osx.md

@@ -9,3 +9,13 @@ or environment variables.
 
 For up-to-date information on building, installation, and bundling, see the
 [GTK website](https://www.gtk.org/docs/installations/macos).
+
+## Content types
+
+While GTK uses MIME types, macOS uses Unified Type descriptors.
+GTK maps MIME to UTI types.
+
+If you create a macOS app for your application, you can provide
+custom UTI/MIME types mappings in the
+[Information Property List](https://developer.apple.com/documentation/bundleresources/information_property_list)
+for your application.