code: public static String addUtf8Charset(String contentType)
      {
          if (contentType == null)
          {
              return "text/plain;charset=utf-8";
          }
  
          Matcher matcher = CHARSET_PATT.matcher(contentType);
          if (matcher.find())
          {
              // replace what ever content-type with utf8
              return contentType.substring(0, matcher.start(1)) + "utf-8" + contentType.substring(matcher.end(1));
          }
  
          return contentType + ";charset=utf-8";
      }
docs: Adds the utf-8 charset to a content type.
  
  @param contentType
  @return the {@code contentType} with {@literal ;charset=utf-8} appended.