code: 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)
  }
docs: // SetHeader sets the Content-Type for a given ResponseWriter and filename extension