Gorilla Mux处理任何模式，但静态FileServer除外

I try to Handle any pattern another than "/app/*" to FileServer Handler. I use mux router to handle "/app/*" urls, and than I need to handle any others urls to static FileServer.This is my code:

r := mux.NewRouter()
r.Handle("/app/deleteComment", c.Handler(PreHandler(DeleteCommentHandler)))
r.Handle("/app/adminDeleteComment", c.Handler(PreHandler(AdminDeleteCommentHandler)))
(...)

fsa := justFilesFilesystem{http.Dir("build/")}
http.Handle("/", http.StripPrefix("/", http.FileServer(fsa)))

And now it works with pattern "/" but i want it to works with any pattern diffrent then "/app/*". I try it many ways but i think there is a simple solution how to handle it. Please help. Cheers.