router := mux.NewRouter()
router.HandleFunc("/file/{token}/serve", fileHandler.FileServeHandler()).Methods("GET")
...
...
...
func (fh *fileHandler) FileServeHandler() http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        vars := mux.Vars(r)
        token := vars["token"]

        // find file record by token from db
        file := fileService.FindByToken(token)

        // save file to a folder
        filePath := "./files/" + strconv.Itoa(file.ID) + file.ext
        ioutil.WriteFile(filePath, file.Binary, 0644)

        // now, i have to serve it to the user for download ???

    }
}