I think this is in the “then don’t do that” category. Most sites restrict uploads to particular file extensions. Most humans don’t create files with no extension.
I have some lab acquisition equipment that outputs files without extension. Can’t control how their hardware/software works…
It might be the status quo to always include an extension but the fact is that all common OSs support filenames without extension so I think it make sense that Discourse would support it as well. I know this bug won’t affect 99.999% of users so no hard feelings if it doesn’t get fixed