MySQL Shell installing customer plugins

[IGG t]

I have been trying to add a plugin to MySQL Shell (8.0.17), but it is refusing to see it exists.

On my Windows 10 PC I have created the following directories.

%appdata%/MySQL/mysqlsh/plugins/ext/demo

In the /demo directory I have added the following files:

init.py
testPlugin.py

The init.py file contains:

from ext.mysqlsh_plugins_common import register_plugin
from ext.demo import testPLugin as testPlugin

I close all Shell windows, and open a new one. And then run

\h ext.demo

But all I get is:

No help items found matching ext.demo

If I try and run:

ext.demo.testPlugin()

I get:

ReferenceError: ext is not defined

Am I missing something here?