Hi,
Could you show me your Dovecot configuration? My installation is from iRedMail, and I'm worried that I might have something set up that's causing this effect with the new iOS. I would like to compare and see what might be different between a working system.
There's a slight difference between my test environment, where the session is closed, and upon opening a new session, the attachment is loaded. In the production environment, the session isn't closed, but loading the attachment takes about the same time as in the test environment, around 90 seconds. All this happens within the same open session.
The difference between the test machine and the production one lies in the certificates. The test one uses a self-signed certificate, while the production one uses a Let's Encrypt certificate.