I noticed that GMail is warning about missing attachments and heard that Thunderbird does this, too, from Arber Borix, who also responded to a request for a screenshot (below). Thanks, Arber!
Thunderbird: Found an attachment keyword: attached. Add Attachment... Remind Me Later
Thunderbird is, in fact, getting better. Its add-ons could make it comparable to GMail in terms of organizational power, I’d suppose.