Creating a Smart Group for Markdown Files

I’ve just started working with the Markdown format, and wanted to create a smart group that would show all the Markdown files (similar to the smart group “All PDF Docs”). However “Markdown” isn’t shown as one of the available values for the “kind” attribute. How can I create a smart group for Markdown files?

Markdown are just plain text files, so use that option.
You can add a filename ends with criterion too, if desired.