Attachment only for premium

Yes!

add_filter('bp_better_messages_user_can_upload_files', function( $can_upload, $user_id, $thread_id ){
 
return $can_upload;
}, 10, 3);

Thanks a lot so far ?? I am a layperson, so: can I just insert this into the child theme? Is this compatible with any membership plugin or do I have to adapt?

You can insert this to child theme, but this code is not complete.

You need to add your logic still based on membership plugin functions you are using and return false when you want to disable files for the user.

oK, so I cannot do it by myself :-/ But Thanks anyway.