Why are static files not opening?

Question

go

app
   cmd
      myapp
         main.go
   web
      static
         css/
         img/

type Server struct {
       router *mux.Router
             ....
       }

  s.router.Handle("/static/", http.FileServer(http.Dir("web/static/")))
  s.router.HandleFunc("/", s.handleIndex())

Doesn't want to open (for example) static/img/favicon.ico = 404. Checking if the folder exists passes. What am I doing wrong ?