FWIW, the IANA media type registry [1] lists "File extension(s)" under "Additional information" for some of the media types, so it may make sense to speak of filename extensions associated with registered media types. Though given the initial odd wording, it could be anything.

Edit: as for what's actually used, looks like [2] it's the ruby mime-types gem [3], which is based on both IANA registry and various other recommendations [4], AIUI.

[1] https://www.iana.org/assignments/media-types/media-types.xht...

[2] https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65954/...

[3] https://rubygems.org/gems/mime-types

[4] https://github.com/mime-types/mime-types-data

Uhh, I'm not sure where you see the word "extension" on your [1], but I don't.

Registrants can include info like file extensions with their registration, but those extensions are not registered (not by IANA, anyway).

That "[1]" was for the registry, the extensions are listed for particular media types, such as application/xml [1]. Also mentioned in RFC 6838 section 4.12 [2].

And indeed, what IANA registers there is media types, so I've only mentioned associations. I think it still works for a-dub's argument, that it's obvious (though I'd word it differently, such as it being a reasonable guess) that if "MIME type format" is mentioned and "mov" is shown as an example, what's actually meant is filename extensions associated with registered and/or otherwise known media types (which turned out to be the case).

[1] https://www.iana.org/assignments/media-types/application/xml

[2] https://www.rfc-editor.org/rfc/rfc6838.html#section-4.12

But "mov" is not shown as an example. The particular requestor in this issue wanted to use mov, however no example was shown in the error message. Moreover even in the requestor's case the user has to already be savvy enough to guess that "mov" is the part of "isaac.asimov" that it had a problem with, which is not at all obvious.

I don't know exactly what list GitLab is using (because, as has been mentioned, there's no central registry of file extensions), but there are many extensions more obscure and less obvious than even mov, which would further obfuscate the actual problem.

Officially no such registry exists. In practice, Apache does have such a registry by default: https://svn.apache.org/repos/asf/httpd/httpd/trunk/docs/conf... and other systems do use that mapping or a similar one.

> File extensions exist completely outside of MIME.

Yes. I am well aware that certain software, such as Windows, MimeMagic, or Apache, do include their own lists. That is not a "registry" and in fact you will likely find that every such list is either identical to, a fork of, or incompatible with every other such list.

I agree with you, I think?

Whether you call such a list a registry or not is fairly trivial, I think. Their web server uses such a mapping, and for some reason usernames with suffixes on that list confused something. Call it a registry, or a mapping, or a list, the meaning of the statement is the same, and I don't think it prevents most people from understanding what's going on.

registry (noun): a place or office where registers or records are kept; an official list or register.

In this case, a registry would be a mapping, however a mapping is not a registry. A registry is the single, centralized place from which you can look up registrations, to guarantee that there are no conflicting records. Literally none of that is (or can be, at this point) true of file extensions.