This is an ongoing issue even with the latest Thunderbird 102.x where attachments 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.
1. Click on "Settings". then go to "General".
2. Scroll to the bottom to find "Config Editor".
3. Search for "mail.
After that your attachments will download properly.
References: https://support.mozilla.org/en-US/questions/1268926
thunderbird, attachment, download, corrupt, filesize, bytes, solutionthis, ongoing, attachments, interruption, workflow, invalid, deleted, quot, settings, scroll, config, imap, mime_parts_on_demand, references, https, mozilla, org, en,