To make Thunderbird download attachments quicker, and to make it stop fucking corrupting downloads, go to the config (Options -> Advanced -> Config Editor), search for “chunks”, and change “mail.server.default.fetch_by_chunks” from true to false. Restart Thunderbird.
This wont change messages already downloaded (attachments will still be fucked). I deleted my local cache of mail and restarted so it would re-download the messages.
Attachments seemed to download a lot faster, and attachments have been opening fine.