[IMP] mail, test_mail: simplify attachment

in `_message_read_dict_postprocess`.

This makes the method easier to follow and does not increase the query count.

It actually reduces it when the attachment data were already in the cache, by
not always reading them again.

The `has_access_to_model` is removed and replaced by `sudo` because:
- from portal everything is sudo so it's pointless to check access on top of it
- from backend, it's almost impossible to trigger the case, and it's not like
  returning is_main True would leak any sensitive information to an employee

Part of task-2180311
PR: #43841

0 files changed, 0 insertions, 0 deletions