Make MIME type / extension guessers extendable

I was working on https://drupal.org/node/1921558 trying to switch our mimetype retrieval system to the Symfony one which is a lot nicer, and i had to override certain things..eg the hardcoded registration of checkers on the constructor.
But self:: and private made me copy/paste more stuff than needed.

Could we switch to static:: and protected instead? Or i am doing something fundamentally wrong extending those classes?

Thanks