registry (noun): a place or office where registers or records are kept; an official list or register. In this case, a registry would be a mapping, however a ma

registry (noun): a place or office where registers or records are kept; an official list or register. In this case, a registry would be a mapping, however a mapping is not a registry. A registry is the single, centralized place from which you can look up registrations, to guarantee that there are no conflicting records. Literally none of that is (or can be, at this point) true of file extensions.