code: public void postMultipart(String url, Map<String,String> map)
      throws Exception
    {
      String boundaryStr = "-----boundary0";
  
      StringBuilder sb = new StringBuilder();
  
      map.forEach((k, v) -> {
        sb.append(boundaryStr + "\r");
  
        sb.append("Content-Disposition: form-data; name=\"" + k + "\"\r");
        sb.append("\r");
        sb.append(v);
        sb.append("\r");
      });
  
      String request = "POST "
                       + url
                       + " HTTP/1.0\r"
                       + "Content-Type: multipart/form-data; boundary="
                       + boundaryStr
                       + "\r"
                       + "Content-Length: "
                       + sb.length()
                       + "\r"
                       + "\r"
                       + sb;
  
      request(request, null);
    }
docs: Performs multipart post
  
  @param url
  @param map
  @throws Exception