@SSOLER yes that is the existing core functionality. we cannot enable/disable detach for specific user for a specific object (LU) only.
Not sure whether it helps but pls check following post to see if it gives some idea.
Disabling the 'Detach' option in Document management Conditionally | IFS Community
I want to add that you should only make document attachments available for object types/LUs that you need them to be available to. But once you can connect a document to, say, a sales contract, any user who can connect documents can do that, from any screen where such objects can be seen. If some user should not be able to connect a document to a certain object type, a custom event can stop that.