This is really bizarre.
Thanks, AdvancedSetup, for deleting the attachment.
What's bizarre is that with no contents to the file, it displayed a trash can to the right of it, so that I could delete it. None of my other attachments displays such an icon, including the screenshot from my last message.
Cycling through all the sorting and filtering options didn't result in a trashcan appearing, and I'm not seeing anything within my settings that relates to being able to delete attachments.
I'm going to try editing my previous message, to see if it gives me the option of deleting the screenshot attached to it.
Well, it didn't give me that option.