[Arnt]

BTW there's a reason for that. The IETF decided (it must be a couple of decades ago) to restrict itself to matters of the internet. Things like file formats are thus out of scope for RFCs. There have been exceptions, RFC5952 is a good example and I know at least two others, but by and large RFCs are about the internet now, not about file formats or other worthy subjects.

[Alupis]

RFC 6120 and 6121 are for XMPP (chat), and define XML is to be used, and even goes into the exact structure of the XML "packets".

[zeen]

Bad example. XML is the wire format in XMPP RFCs, not a file format.

[nly]

RFC7159? It even recommends a file extension.

[Arnt]

That's a perfect example: Publishable as an RFC because the format is used in many APIs on the general internet, but also says a little about a local matter, in this case the file names.

Is the rule a bit messy? Yes it is!