mime.go
func (mr *Reader) SetHeader(w http.ResponseWriter, ext string) {
	mimestring := mr.Get(ext)
	if mimestring == "" {
		// Default mime type
		mimestring = "application/octet-stream"
	}
	if mr.utf8 {
		mimestring += "; charset=utf-8"
	}
	w.Header().Add("Content-Type", mimestring)
}
func (mr *Reader) SetHeader(w http.ResponseWriter, ext string) {
	mimestring := mr.Get(ext)
	if mimestring == "" {
		// Default mime type
		mimestring = "application/octet-stream"
	}
	if mr.utf8 {
		mimestring += "; charset=utf-8"
	}
	w.Header().Add("Content-Type", mimestring)
}
// SetHeader sets the Content-Type for a given ResponseWriter and filename extension