Browsing artifacts in pages does not work for xhtml files?

Summary

Browsing artifacts in pages (open in browser) does not work for some files. Is it based on extension?

Steps to reproduce

Create both .html and .xhtml files in artifacts

Example Project

https://gitlab.com/Molcas/OpenMolcas/-/jobs/347383517/artifacts/browse

(The artifacts are removed in 24 hours, but there will probably be some other available job in the project, any doc job will do.)

What is the current bug behavior?

I can open some files in the browser (e.g. build/doc/html/index.html) but not others (e.g. doc/doxygen/html/index.xhtml).

What is the expected correct behavior?

I expect both (all?) files to be openable, or at least for the requirements to be clearly documented.

Output of checks

This bug happens on GitLab.com