Is archetype file extension hardcoded?

I noticed today that if I change the archetypes/ extension to .markdown for example, new content created via hugo new post/first-post.markdown doesn’t inherit the front matter specified in the archetype, and only gets the defaults.

I tried different file extensions, none work except for no extension at all which works fine, just as .md.

I believe that files in archetypes shouldn’t have restricted extensions since for content everybody can use whatever they wish: .markdown, .mkd, .md and so on, or am I the only one who cares about this?

I don’t know that code path, so it would be faster if you looked yourself.

A pretty valid workaround would be to set markdown = mmark or similar in the archetype frontmatter.