This document has long been obsolete because Mozilla got a clue eventually and made this part of the options in the user interface; this remains only for the purpose of history.

While this is documented elsewhere I never bothered to look in to it. Perhaps that is because it annoyed me and I just wanted to ignore it. About 10 minutes ago I was looking for something else in Thunderbird configurations and then I remembered that just like Firefox, Thunderbird has an about:config page.

So, here's how you get rid of that bloody reminder for good :

1. In the Thunderbird main menu navigate to Edit -> Preferences ->.
2. In the Preferences area, navigate to the Advanced options.
3. In the Advanced area navigate to the General tab.
4. Click 'Config Editor...' at the bottom of the General section of Advanced options.
5. Thunderbird will warn you that editing advanced options might "void your warranty" (whatever warranty that is, is beyond me). Click the button below labelled "I'll be careful, I promise!"
6. In the search box type 'attach' to search for the settings you need to edit.
7. You should see several settings and the ones you need to change are:

   mail.compose.attachment_reminder
   mail.compose.attachment_reminder_aggressive
   mail.compose.attachment_reminder_keywords

For the first two you can simply double-click the name - this toggles the settings to false. The third you still double click but you will get a pop-up where you can edit the keywords. You can clear it or edit as you like. If you want it back to the defaults just right click (instead of double clicking) and then left click 'Reset' in the menu. The reset option works for the others, too.

That's all there is to it. If you then realise you have the problem where you write you will be attaching something and then you click 'Send' before attaching the file, you can reset the options back to their defaults (Unless, of course, you forget why you no longer get reminders. In that case I would have to wonder why you tried this in the first place, though).