reflect.Value.Convert: value of type reflect.Value cannot be converted to type http.FileSystem

I meet the question that
```
reflect.Value.Convert: value of type reflect.Value cannot be converted to type http.FileSystem
```
which was seemd caused by
```go
http.Handle("/js/", http.StripPrefix("/js/", http.FileServer(http.Dir("js"))))
```
by the way it's running in goland

├── static
│   └── js
│   └── index.js
├── templates
│   └── index.html