POCO : How to upload image to webser using poco in c++

I am trying to upload an image to remote web server. I have used HTMLForm and FilePartSource. I am able to successfully upload image to local sever (i.e. loclhost) but when i try to upload it in remote server, the response received from remote web server is "411 Length Required ". I tried to set request.setContentLength(sizeofimagefile) but still same issue. Can anyone guide me on what is the issue or . Here is my code.

    HTMLForm htmlform;
htmlform.set("aaaaaa", "bbbbbbb");
htmlform.set("cccccc", "ddddddd");
htmlform.setEncoding(HTMLForm::ENCODING_MULTIPART);

PartSource * pFileSrc = new FilePartSource("filename", "application/octet-stream");

std::istream& mystream = pFileSrc->stream();
mystream.seekg(0, std::ios::end);
int uiLength = mystream.tellg();

    htmlform.addPart("file", pFileSrc);

URI uri("yyy");

    HTTPClientSession session(uri.getHost(), uri.getPort());        
HTTPRequest post_req(Poco::Net::HTTPRequest::HTTP_POST,"xxx",HTTPMessage::HTTP_1_1);        
post_req.setKeepAlive(true);
htmlform.prepareSubmit(post_req);


std::ostream& oustr = session.sendRequest(post_req);
htmlform.write(oustr);

HTTPResponse res;
std::istream& rs = session.receiveResponse(res);

std::cerr << rs.rdbuf(); 

Thanks in advance