Permission conflict: group permissions vs. group forum permissions

Scenario:

• group "registered users" has the "group forum permission" in a single forum for: "can attach files".

There is no never, which will overwrite this, so users in that group should be able to attach files to their posts. File attachments are allowed and a file-group is allowed in postings.

"View forum-based permissions" > "Forum A" > "User A" > Content > Can attach files > YES
Shows:

• Default: No; No
• Registered users: YES: YES
• User A: No; YES
• Result: Yes

But in fact, the user does not have the "tab: attachments" in the posting editor.

Additional this is needed:

• Permissions > Group permissions > Registered users > Select type: User permissions > Tab: post > Can attach files: YES (with NO, it does not work!)

Question:

• why is the second step still needed?
• why overwrites the NO in "group permissions" the YES?
• why is it not displayed in "forum-based permission"?

Of course it is no forum-based permission, but the result should be still displayed as "NO" and with it, why it is NO.

I would rather expect, that the YES leads to a YES, when there is only a NO standing against it.