Searching by excluded extension only excludes the first extension

I expected https://gitlab.com/search?utf8=%E2%9C%93&snippets=false&scope=blobs&repository_ref=&search=http_requests_total+code+-extension%3Amd+-filename%3A*_spec.rb+-extension%3Aprom+-extension%3Aout+-extension%3Ayml&group_id=9970 to exclude the md, prom, out, and yml extensions. It only excludes the md extension:

If we do only support a single exclusion, it would be good to document that at https://docs.gitlab.com/ee/user/search/advanced_search_syntax.html#excluding-filters.