"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.)"

https://daringfireball.net/linked/2014/01/08/markdown-extens...

Thanks for finding this post. I did a quick search for it and came up empty.