Permission when editing post containing attachment of user without permissions in subforum

As admin cannot edit a post containing an attachment in a certain subforum when the user who posted the post has no longer access to that specific subforum.

In my opinion this is a bug, since admins with every permission should overrule any old regular user's permisisons.

Reproduce:

1. login as admin
2. make sure you have every permission on the admin account there is
3. create user account X
4. create private subforum Y
5. give X permissions to view / post etc. in Y
6. login with X
7. post message with attachment in Y
8. logout X
9. login as admin
10. remove X's permissions to view / post etc. in Y
11. edit X's post in Y, e.g. change some text
12. receive error message 

This only occurs when the post contains an attachment.

Error: parse_message->get_submitted_attachment_data()