AttachmentReminder is quite possibly one of the most useful extensions for Thunderbird. What it does is allows you to add keywords that relate to attachments and, on send, if it finds one of those keywords, but no attachment, it’ll warn you.
I simply have attach in AttachmentReminder’s keyword list and anytime any word contains attach, or any version of that word, it’ll pop-up and ask me if I was supposed to attach something.
This is so handy as I have a tendency to send emails and forget about the attachment, even though I said something was attached. The version number is quite low, but it’s still one of the most useful Thunderbird extensions!