tl;dr: `text/markdown` since March 2016
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In March 2016, `text/markdown` was registered as [RFC7763 at IETF](https://tools.ietf.org/html/rfc7763).

Previously, it should have been `text/x-markdown`. The text below describes the situation before March 2016, when RFC7763 was still a draft.

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There is no official recommendation on [Gruber’s definition](http://daringfireball.net/projects/markdown/), but the topic was discussed quite heavily on the [official mailing-list](http://six.pairlist.net/pipermail/markdown-discuss/2007-June/thread.html#640), and reached the choice of `text/x-markdown`.

This conclusion was [challenged later](http://six.pairlist.net/pipermail/markdown-discuss/2008-February/000960.html), has been confirmed and can be, IMO, considered consensus.

This is the only logical conclusion in the lack of an official mime type: `text/` will provide proper default almost everywhere, `x-` because we're not using an official type, `markdown` and not `gruber.` or whatever because the type is now so common.

There are still [unknowns](http://six.pairlist.net/pipermail/markdown-discuss/2007-June/000652.html) regarding the different “flavors” of Markdown, though. I guess someone should register an official type, which is supposedly [easy](http://tools.ietf.org/html/rfc4288#section-3.4), but I doubt anyone dares do it beyond John Gruber, as he very recently [proved](http://blog.codinghorror.com/standard-markdown-is-now-common-markdown/) his attachment to Markdown.

There is a [draft](https://datatracker.ietf.org/doc/draft-ietf-appsawg-text-markdown/) on the IETF for `text/markdown`, but the contents do not seem to describe Markdown at all, so I wouldn't use it until it gets more complete.

Closes #4999

When submitting the previous commit, I had not noticed that the parameter allowAtxHeaderWithoutSpace had type Object instead of type boolean. I've fixed this now.

Adding documentation for the `allowAtxHeaderWithoutSpace` configuration parameter.

Issue #4960

Issue #4960

Closes #4960

Closes #4947

Issue #4864

See https://discuss.codemirror.net/t/gfm-mode-doesnt-seem-to-respect-codeblock-mode-specific-indentation/1286

Closes #4773

Issue #4735

The original character class `[^\\\[\\]]` escapes `\\` instead of `\]`
(e.g. it would be the character class `[^\\\[]` followed by a `]`
which I think is wrong).

Closes #4686

Closes #4628

As per http://spec.commonmark.org/0.27/#html-blocks

Issue #4611

Closes #4568

Issue #4528

Closes #4528

Closes #4484

Issue #4426