AFAIK email is imported in the mbox file format (or at least it can be), which is basically a file with the e-mail headers and bodies in sequence. Any attachments are stored as text as well and decoded back into a file when imported.
In the latter discussion I proposed a method to @cgrunenberg that would separate attachments from emails, while still keeping a relation with that e-mail. Whether that’s easy or very difficult to do, feasible and necessary is up to the DT staff of course.
Personally I think it would be nice if DT had a standard way to have the attachments somehow available alike regular files, e.g. with the duplication strategy above,