Most wiki formatting is lost

I was merrily editing a page online when suddenly, upon a save, most of the formatting was lost: links are not rendered, headers are not rendered, numbered lists are made massive and bold instead of being numbered

Bulleted lists work fine

I clicked edit, copied the content, pasted verbatim into a new page and the new page worked fine

When I used hg to clone the repo I saw the problem page was a different extension to all the others. it was a .md file and the rest were .wiki files

When I renamed it to .wiki and pushed it back it worked fine, but how did it come to be renamed in the first place?