Right now, thunderbird is able to detect potential missing attachments when composing new email, and prompt the user just before sending it.
However, this is not always accurate. When the user clicks send, this prompt prohibits everything from going on. The user may not always realise this prompt after clicking send, such as when hurrying up other stuffs and switched window immediately after clicking send. This prompt dialog window will result in email not being sent.
(Actually I have experienced this quite several times now. Some were important emails not being sent for the whole workday -- I composed them at home in the morning, and went out to office.)
Thus, it would be ideal to support setting a timeout for this prompt window. After timeout, Thunderbird should assume there is no missing attachment, and send the email out anyway.
Those who still want it to wait indefinitely can set this to 0, while those experiencing issues can set a number for it (e.g. 30min).