I finally managed to find the run-mailcap binary (mime-types package),
but I'm still getting issues with rendering HTML email properly. I get
the error in the subject line as well as another (can't reproduce
without quitting sup).

I wonder if anyone might share their .mailcap file, or suggest any
alternative methods for stripping out the nonsense HTML markup and just
leaving me with plain text.

Thank you,
Christer