This is an ongoing issue even with the latest Thunderbird 102.x where attachments downloaded via IMAP just won't save or will be corrupt which is a huge interruption to workflow or if you come back later to find out the file you thought you saved is invalid/corrupt and you have perhaps deleted the e-mail.
How to solve the Thunderbird filesize attachment issue?
1. Click on "Settings". then go to "General".
2. Scroll to the bottom to find "Conf........