Sorry, I've been trying to download from Tutanota mail and you can't download files from Ut, you don't do it with your browser and neither from webapp,
The problem is from Tutanota, I have 4 different emails and they all download the files without problems,
Gmail, Protonmail, Mailbox and Mailo,
We have an option, use Dekko for Tutanota mail and verify that it works
Regards....