Custom http.FileServer to overwrite Content-Type header.

custom-file-server.go

package main

import (
	"net/http"
	"strings"
)

var CustomTypeMap = map[string]string{
	"text/x-java-source": "text/plain",
}

type CustomResponseWriter struct {
	w http.ResponseWriter
}

func (cw CustomResponseWriter) Header() http.Header {
	return cw.w.Header()
}

func (cw CustomResponseWriter) Write(b []byte) (int, error) {
	return cw.w.Write(b)
}

func (cw CustomResponseWriter) WriteHeader(code int) {
	contentType := cw.Header().Get("Content-Type")
	for k, v := range CustomTypeMap {
		if contentType == k || strings.HasPrefix(contentType, k + ";") {
			cw.Header().Set("Content-Type", strings.Replace(contentType, k, v, 1))
			break
		}
	}

	cw.w.WriteHeader(code)
}

func CustomFileServer(h http.Handler) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		h.ServeHTTP(CustomResponseWriter{w}, r)
	}
}

func main() {
	http.Handle("/", CustomFileServer(http.FileServer(http.Dir("public"))))
	http.ListenAndServe(":8080", nil)
}