This is a good call. I know it's been suggested multiple times over the years; I wonder what the rationale was for rejecting the format, or at least having the option to render a file when it's loaded. (Maybe a "display as HTML" button or the like would be required before it would be rendered.)
“Markdown” is a family of writing formats. There is no one “Markdown”. It’s completely unsuitable for direct inclusion in the web platform.
Related reading: https://www.rfc-editor.org/rfc/rfc7763.html text/markdown registration.