YAML media type?

Ruby on Rails uses application/x-yaml with an alternative of text/yaml (source).

I think it's just a matter of convention, there is no technical why, as far as I can tell.

I'd say text/x-yaml:

text over application because it's a human-readable

x-yaml over yaml because it hasn't been accepted into the registered list of mime types.

Edit: from RFC 3023 (XML Media Types):

The top-level media type "text" has some restrictions on MIME entities and they are described in [RFC2045] and [RFC2046]. In particular, the UTF-16 family, UCS-4, and UTF-32 are not allowed (except over HTTP[RFC2616], which uses a MIME-like mechanism).

Interesting... Not exactly sure what it means, but food for thought.

Although another answer was accepted, please refer to this Proposed media type registration for YAML thread on the IANA mailing list for reviewing Media Type in which Ben Harris, University of Cambridge Information Services, proposed in July 2015 on behalf of the YAML team the media type:

text/vnd.yaml

with (suggested) deprecated aliases:

text/yaml
text/x-yaml
application/x-yaml

That is still proposed/pending (the thread does not indicate status of the proposal) so this answer is no more definitive than the others :-)