Actually, the software running this forum -
Mailman (not Listserv) - should have no problem with attachments. The Mailman system tries to be careful about spam and so on, and therefore there are filters that reject the "wrong" kinds of attachment. Maybe these settings are too sensitive, so to speak, and the attachment looked wrong to Mailman for some reason. I'll look into it.
I've noticed that attachments don't get saved in the archive of messages. I can't see a way to make the attachments persist into the archive, and it's a pity. Though I can understand why an archive might be designed that way.