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...
Still wish Gmail had it, though.
EDIT: See below. It was an issue unique to my account
Here's a blog post by someone digging into how it works: https://www.labnol.org/internet/gmail-attachment-reminder/31...
I tested it with my work email on Google Workspace, and it showed the popup. It must be a setting.
Weird.
EDIT: After toggling experimental features on, then off again, it now works. ¯\(º_o)/¯
Still the most sure-fire way of fixing anything computerized.
A quick search says that automatic footers (company logos) can be interpreted as attachments...meaning you won't see the warning. Could that be your issue?
https://www.lifewire.com/how-to-get-a-forgotten-attachment-r...