It just displays .md files. Plain and simple – exactly what I searched for (there are MD editors with far more bells and whistles, if you want to, this one is totally minimalist).
Unfortunately, Firefox (at least under openSUSE Linux) has an issue with the MIME type. The solution linked here solves it: https://github.com/KeithLRobertson/markdown-viewer/issues/2
Oh, and btw, it only works (for me) with UTF-16, not UTF-8 as long as you don't manually add it a BOM, which is somewhat unusual … (can be fixed with " sed -i '1s/^/\xef\xbb\xbf/' foo ")