code: func ContentTypeByFilename(filename string) string {
  	dot := strings.LastIndex(filename, ".")
  	if dot == -1 || dot+1 >= len(filename) {
  		return DefaultFileContentType
  	}
  
  	extension := filename[dot+1:]
  	contentType := mimeConfig.StringDefault(extension, "")
  	if contentType == "" {
  		return DefaultFileContentType
  	}
  
  	if strings.HasPrefix(contentType, "text/") {
  		return contentType + "; charset=utf-8"
  	}
  
  	return contentType
  }
docs: // ContentTypeByFilename returns a MIME content type based on the filename's extension.
  // If no appropriate one is found, returns "application/octet-stream" by default.
  // Additionally, specifies the charset as UTF-8 for text/* types.