A minimal version of util.py just for this issue is fine.  What is blocking the issue deciding exactly what the minimum should be.  Are both the extension list and function needed?

Alex says somewhere in PR comments that ispythonsource passes directories.  That seems wrong.  Perhaps moving and revising it and making no-extension files browsable and removing the x.py(?) limitation should be a separate issue.  IDLE users on *nix don't mind using extensions.  I have not seen a complaint, though perhaps people who do mind silently use something else.  I need to check again how the editor function is used and decide if we can leave it alone for now.  So don't remove anything just now.

I need to experiment with Ron's suggestion on my Mac.

The Windows context menu is definitely a separate issue.

Please add idle_test/example.pyi with the line in msg403741 for manual testing now and possibly future automatic testing.