Newer versions of Thunderbird have built in PGP capabilities. Normal encrypted mail works fine, however file attachments are broken.
When opening the file, it says 'null undefined', which looks like this:

And then, when I actually open it:

And it's not just me, I learned about this from this support thread: https://mail.google.com/mail/u/human@flowcrypt.com/#inbox/FMfcgxwKjxHWSvfTzMnjlSkknmltppDt