Too late now, I suppose, but the only file extension I would endorse is “.markdown”…
(I personally use “.text” for my own files, and have BBEdit set to use Markdown syntax coloring for that extension, which is why I never saw a need to endorse an official extension.)
in response to Hilton Lipschitz's post, which says in part:
The correct file extension for markdown files is: .markdown
and I think both would agree with Randall Munroe's assessment of the relative trustworthiness of various File Extensions.