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.