Preferred filename extension is …?

Gruber writes:

the only file extension I would endorse is “.markdown”

Of course, given that he forbids the project to use that name, we can’t follow his advice here. So what file extension should we use for our CommonMark text files? .commonmark?

(Apologies if you have already covered this topic, but I searched for it both here and on Google and didn’t find the answer.)

The existing topic: File extension for CommonMark?