静态文件Go网络应用始终返回404

I have a very basic Go web app which is simply supposed to serve static files from a public sub-folder:

package main

import (
"net/http"
)

func main() {
    mux := http.NewServeMux()
    fs := http.FileServer(http.Dir("public"))
    mux.Handle("/", fs)
    http.ListenAndServe(":8181", mux)
}

If I run the app (go run main.go) it starts, but when I visit

localhost:8181/readme.txt

or

localhost:8181/index.html

both of which exist within the public sub folder, I get a 404 error returned. Nothing is logged (that I know of), but I’m at odds to understand why it is not being served.

Any suggestions welcomed.

Thanks