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.