I just found the problem … and it turns out that working on this way too late at night was the problem. It’s not a Postie issue at all. My script that sent that message didn’t send the right file. That’s yet another of those problems that can be caused by the three brain tumors from my first cancer. It’s one of those highly annoying bits of the aftermath of cancer #1 that I’ve just had to learn to live with.
Apologies for the waste of everyone’s time. I couldn’t see it (literally – I saw what was supposed to be there, not what was) until just now, when the damage from the tumor that was in my occipital lobe finally let me see what was really there. Problem solved (confirmed).