I was going to build a plugin to support new kinds of media in your notes. . Digging around the api I don’t see a way to register a mapping between extension and factory=> Element.
I can see where this is happening in app.js there is logic that has a set of arrays. An array per embeddable content type. The content of the array is the extensions for that content type, Example mp4, webm, ogv for the array for audio. Then it finds the type that matches and has logic that generates a set of elements to represent that content in the preview.