Whether you call such a list a registry or not is fairly trivial, I think. Their web server uses such a mapping, and for some reason usernames with suffixes on that list confused something. Call it a registry, or a mapping, or a list, the meaning of the statement is the same, and I don't think it prevents most people from understanding what's going on.
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.
Whether you call such a list a registry or not is fairly trivial, I think. Their web server uses such a mapping, and for some reason usernames with suffixes on that list confused something. Call it a registry, or a mapping, or a list, the meaning of the statement is the same, and I don't think it prevents most people from understanding what's going on.