The correct file extension for markdown files is:

.markdown

That is all. No others.

Make it so!

Update: It seems the GitHub .md extension is becoming canonical for those who prefer shorter file names. And Dr Drang @drdrang uses it. Who am I to argue. But I’ll be sticking with .markdown.

I suppose I’d better say why:

• We no longer live in a 8.3 world, so we should be using the most descriptive file extensions. It’s sad that all our operating systems rely on this stupid convention instead of the better creator code or a metadata model, but great that they now support longer file extensions.
• Markdown is text but text is not markdown. It’s a few seconds work to associate an application to edit text and another to edit markdown files, so use that. I use different editors for them, so its best to help the operating system here.
• (No longer applicable) .md (a.k.a the GitHub extension) could be mistaken for GCC Machine Description files, confusion is bad, don’t do it.
• All the others are just shortened forms on the canonical .markdown, so use the canon. Avoid .mdown, .mkdn, .mdn or even the horrible .mdtext.
• If you have made the mistake in the past of using the wrong file extension, write a script to rename the old files now. Ok, now run it. Good. Problem solved.