I think you miss the point… they still can’t be opened on-device because Sailfish Office does not support markdown (not even as plaintext ignoring formatting).
Implementing a markdown renderer there could be interesting. There are several possibilities to do it:
using RichText from QML Label (markdown is natively supported since Qt5.14… no comment on Qt version), after a tiny markdown to HTML change on the fly,
using Caligra, if it happens that there is a markdown converter (didn’t look at, but there might be),
using a md to PDF conversion tool, if any exists and then use the current PDF renderer.
One more question, how comes the mail app takes forever to download atachments from a gmail address?
Also sync ing the mails from the gmail account is slow as it used to be. A regression perhaps?