Permalink do not work for my post

I have created a post with a new keyword in categories.

Pointing my browser at http://localhost:4000, I clicked on the link of the post, and the browser wanted to save my post instead of displaying it.

Whan looking into _sites, I have found a my-permalink file and a my-permalink.html file. When I removed the file without html extension, it worked properly.

What could cause this?

Interestingly, I have found an accidentally created file “ű” in my Jekyll root directory. When I removed it and built the site again, the my-permalink file (without an extension) disappeared and the mentioned page did load on any browser.