I would say, that there is no flaw in the regex, but the application of the regex might be wrong in the given Mp3tag environment.
You can change the notation of the file extension by using an action of type 'Replace using Regular Expression' (this feature is special to this action type only).
Better you use the proposed scripting expression together with an action "Format value".
You are acting wrong when working on all tag-fields the same at once, doing so by running the regular expression against the pseudo tag-field _ALL.
There are tag-fields, which should not be touched, for example UNSYNCEDLYRICS or COMMENT.