Out of interest, I had a look at the relevant Thunderbird source: https://searchfox.org/comm-central/source/mail/components/co...

The code simply checks the subject and body text, excluding the signature and any quoted text, for keywords defined in the configuration item mail.compose.attachment_reminder_keywords. The default keywords are: ".doc,.pdf,.xls,.ppt,.rtf,.pps,attachment,attach,attached,attaching,enclosed,CV,cover letter"

This little feature must have made lots of users happy one time or another...