Any Markdown file I create on my system is marked this way.
Update:
I tried to reset all of the file associations in Mac OS X like this: https://superuser.com/a/960819
and that didn’t seem to help.
but later I have checked again and the file info has a normal md now. So I’m still not sure what the root cause is.
This post seems to describe how you could find an unexpected kind string, but now it’s not useful to me: https://superuser.com/a/981808
So, I don’t know if this oddity with the file was related to Obsidian or not, but it’s gone now.