There's a closed pull request on the mblaze repository

Improve attachments handling
https://github.com/leahneukirchen/mblaze/pull/234

Description:
I had to manually explore to understand how to add attachments, the man page was a bit evasive. It also didn't help that a `~` at the beginning of the path isn't caught, so I added the same treatment as outbox parsing.

Thanks for this fantastic piece of software, the joys of building one's own MUA in ~150 lines of shell scripts...