Plugin directories and how to check in custom plugin?

> How do we register custom plugin with LMS the correct way?

See Settings/Information for the various Plugins folders where you can
put your own plugin. Please not it must be Plugins (uppercase first
character, "s" at the end), and NOT the folder inside the cache.

> On RPi / Debian I saved MyCustomConvert plugin in
>
> /var/lib/squeezeboxserver/cache/InstalledPlugins/Plugins/MyCustomConvert

Not this one :-). It's managed by the built-in plugin manager.

> From web interface 'Settings>Advanced>File types' I could now see my
> changes but now I got a message that the server had to restart to use
> the new settings. This before I've done any changes from Web GUI. *After
> restart from the web interface I now find my MyCustomConvert directory
> have been deleted.*

Exactly. Don't mess with the plugins folders inside the cache folder.
There are others you should use.

> Finaly I ended up copying 'MyCustomConvert' to
> /var/lib/squeezeboxserver/Plugins

That's the one to use.

> Are we to understand the '~/InstalledPlugins/Plugins/' directory are
> protected/maintained and ment only for official plugins?

Not the official ones, but those installed by the Plugin manager in LMS.
Those you select from a list and then are downloaded and installed
automatically.

> Will content in '/var/lib/squeezeboxserver/Plugins' survive an LMS
> upgrade?

Yes.