Move attachment type declaration to backend

[miaulalala]

Is your feature request related to a problem? Please describe.

As a intermittent measure, we added the attachment type that is returned from the backend when saving / updating a backend message by looping through all attachments and adding a type local if it isn't set. This should be moved to the backend.

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response