Mozilla Thunderbird displays a missing attachment warning message “Did you forget to add an attachment” as shown below.
This gets displayed when trying to send an email containing certain keywords such as .doc. .xls etc. but with no attachment.
Thunderbird also provides option to disable this missing warning message and this can be done using the Options menu.
Click Tools menu and select Options from the available menu list. In the Options window, navigate to General tab and un mark the checkbox with label as Check for missing attachments.
Click OK to save and confirm the changes.