[bugfix] restore attachments by reverting to jdk8
- SYMPTOMS: attachments showing up as weird encrypted blobs
- CAUSE: unclear, but symptoms only observed on jdk11
- FIX: reverting to jdk8 fixes it (again, unclear why)
(note: jdk8 does not have an installation candidate on buster, and stretch is deprecated now, so we pull jdk8 package from sid repository. not the best long-term solution, but the best one available)