Markup parameter and "custom input format"

I see that content files of extension “mhtml” are ignored and published as is. It’s written. It’s not really necessary for me, but it would be neat.

You can put any file type into your /content directories, but Hugo uses the markup front matter value if set or the file extension (see Markup identifiers in the table below)

But it doesn’t seem to work for files with extensions not already in a certain list ? I hope it’s not already treated somewhere.