使用golang服务器阻止访问文件夹中的文件(prevent access to files in folder with a golang server)

I've a server in golang who handle folder path like that :

fs := http.FileServer(http.Dir("./assets"))
http.Handle("/Images/", fs)
http.ListenAndServe(":8000", nil)

But in this folder there are privates images, and it shouldn't be possible to access files. So how can i secure image access and prevent anybody to access content of folder.

like that for example :