So I had to write my own file server to set the headers manually like:

contentTypeMap := map[string]string{
    ".html": "text/html",
    ".css":  "text/css",
    ".js":   "application/javascript",
}

filepath.Walk("./dist", func(path string, info os.FileInfo, err error) error {
    if err != nil {
        log.Fatalf(err.Error())
    }
    if info.IsDir() {
        return err
    }

    dirPath := filepath.ToSlash(filepath.Dir(path))
    contentType := contentTypeMap[filepath.Ext(info.Name())]
    handlePath := "/" + strings.Join(strings.Split(dirPath, "/")[1:], "/")

    hf := func(w http.ResponseWriter, r *http.Request) {
        w.Header().Add("Content-Type", contentType) // <---- key part
        http.ServeFile(w, r, path)
    }

    if handlePath != "/" {
        handlePath += "/" + info.Name()
    }

    mainRouter.HandleFunc(handlePath, hf)
    return nil
})

(please optimize if the code is bad, I made the solution myself and I tried so many stuff to fit my needs)

Now with that I recevied the correct files with correct headers.

And I couldn't find any solutions to work with custom headers using http.FileServer in http package. And please provide a easy solution if it exists.