In Finder, folder gets icon when given an extension .md, not with other extensions

By accident, I see a strange behaviour when creating a folder with an extension .md

It gets the the same icon as a file with extension .md (the MarkdownLife icon). For other file extension, I do not see this behaviour.

Is this a bug in Finder? Or markDownLife? Can other confirm this issue?

Using:

• OS X Version 10.9.5


• MarkdownLife Version 1.4.0 (20141019)