type logWrapper struct {
        http.Handler
    }

    func (wr logWrapper) ServeHTTP(w http.ResponseWriter, r *http.Request) {
        log.Printf("%s %s %s\n", r.Method, r.URL, time.Now())
        wr.Handler.ServeHTTP(w, r)
    }    

    func main() {

        router := mux.NewRouter()
        s := http.StripPrefix("/static/", http.FileServer(http.Dir("./files/"))) 
        router.HandleFunc("/", Index)
        router.HandleFunc("/login", LogIn)
        router.HandleFunc("/logout", LogOut)   
            FileHandler := http.HandlerFunc(File) 

        router.Handle("/file.html", logWrapper{FileHandler})
        router.PathPrefix("/static/").Handler(s)
        http.Handle("/", router) 

        http.ListenAndServe(":5000", nil) 
    } 

    func Index(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintln(w, "hello")
    }

    func File(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintln(w, "это обрабатывается в обёртке)) ")
    }