Funny that he didn't use the .md extension. Maybe because he started doing it before he expected his markup would merit its own filetype?
More likely because the whole point of Markdown was to be embedded in text, not a freestanding format for an entire document.
"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...