Yeah, I don't get it. They have already done the hard part - the system seems to correctly identify an attachment as a programme or script. It can even tell the difference between eg. a jpeg with executable privileges and a 'usro' from a shell script with with a 'usro'. All they have to do is set giving a "possible program" an executable icon as the overriding rule to decide what icon a file displays.
In "Finder", a disguised script displays any custom icon assigned to the file. In Mail, any icon displays an icon based on its file extension. What an inconsistent mess.
But I consider that a separate issue. In this case, I consider the bug to be strictly a "Mail.app" bug - the warning that is otherwise displayed is missed if "Terminal.app" is already running.