That simply is not correct. There is no basis for it whatsover. All we have is John Gruber being asked what extension should be used and his response was that .markdown is the only one that he would endorse … he didn’t even say he does endorse it. And a personal endorsement by an individual, especially one who has been uninvolved with it for a decade, is in no way “the standard”.
Er, um, what do you think the point of standardizing existing practice is?
John failed to follow Guido when he had the chance, so the authority of his preferred extension should not be a primary consideration.
More to the point, however, is why do we need a default extension at all?
With respect to mime, are we trying to get browsers to render CM directly?
Otherwise, stmd and its progeny certainly don’t depend on the extension. I’m no longer sure about windows, but at least in OSX if you want to use a file extension associated with a specific program, you’re free do.
Regarding the file extension: my draft registers both .md and .markdown. Media type registrations are not authoritative on the topic of file extensions; however, nothing is. Personally I prefer .markdown. I know, however, there are enough README.md (and other) files out there in existence, that we should not pretend about .md these days.
In my experience (using lots of different Markdown-related tools), .md is the most common extension, with .markdown a distant second. The rest of them (.mdown, .mkdn or whatever) I’ve never encountered in the wild (but some of the editors I’ve used support them).
.mark sounds good, though .md is used more often. If commonmark is highly compatible with gruber’s markdown, just stick to .md, if not then try .mark.
Btw, just notice that R statistic program name their fancy styled markdown document as .rmd to indicate that it is a markdown formatted document that has inline executable R snippets.
Whatever extension you choose like .md or .mark, I recommend adding a ‘Recommendation’ for filenames like .R.mark for R or .py.mark for python. So at least the file can be read even if there is no python or R hooks in the CommomMark reader.
For any file claiming to contain Markdown, the .markdown extension is the most readable and logical. The .md extension is most widely used. All other variations (.mdown, .mkdn, .markdn) are neither readable nor widely used, and as there are 49 ways of arranging the letters of the word into an extension, we should ignore those all.
However, to call CommonMark Markdown would be similar to calling LESS and SASS CSS. Yes, the original subset is still supported in the new language, but the new language is so much more.
Unless CommonMark’s development halts today, it will be a very different beast next year. It will support syntaxes and extensions that have never been valid Markdown. Just like how CommonMark has a name that’s distinct from Markdown, I support giving it an extension and MIME-type distinct from Markdown, and I like Sanpi’s proposed.mark extension.
I realize .md is the most common extension, however it’s also used by GCC machine description files and Modula. Vim, for example, only recognizes .md as Markdown in README.md by default [source]. The second most widely supported extension is .markdown. Other extensions are not supported everywhere (for example, the second shortest .mkd is not supported by Bitbucket) and I find them unreadable anyway.
Conclusion: In my opinion, .markdown should be the officially endorsed standard, but tools should probably also support at least .md where feasible.
Syntax highlighters need either an extension or a hashbang, else they have to resort to more complicated heuristics.
A common convention on Unix/Linux systems is for executable files to have a hashbang and no extension, and for libraries to have an extension (.pl, .py, .rb, etc.) and no hashbang.
You can’t really have extensionless files on Windows; for example, README will be recognized as a README file.
Therefore, Markdown certainly does need an extension. Now, I’m not saying there should be just “one true extension”, but a standard would be certainly nice—one that makes sense, i.e. not something like .text.
Is it possible to also use the ‘naturalization argument’? That the majority of markdown files out there in the wild is already named “.md”, so any files we read should be in “.md” (if we are strictly compatible with majority of markdown formatted files).
If we are not strictly compatible with majority of markdown files, then we should use .cm or .mark, or .mk. Thought an argument can be made that .md is a generic file extension for lightly marked up text, and thus is not markdown specific.
That might be the way to go and we could also to look at the extension apps save Markdown files in. I personally use iA Writer for Mac which saves files with the .md extension by default (but recognises other extensions like .markdown). We could do a comparison of apps which read and write Markdown files and list what file extensions are used.