Make sure extensions start with a dot

In nbconvert, it appears the filename extensions have to start with a dot. Mostly, I think this was changed in #6978, but these two files were missed.

I was thinking of ways to try to avoid this more generally -- maybe a specific `Extension` traitlet that makes sure it starts with a dot, or something? Or, is there a way to validate existing traitlets without creating a new traitlet class?