Вот небольшой кусок кода. Может кому-то поможет
QString boundary("----WebKitFormBoundary");
QByteArray data;
// №1
data += "--" + boundary + "\r\n";
data += "Content-Disposition: form-data; name=\"subject\"\r\n\r\n";
data += "185/55 R16 Continental ContiPremiumContact\r\n";
// №2
data += "--" + boundary + "\r\n";
data += "Content-Disposition: form-data; name=\"bulletin_type\"\r\n\r\n";
data += "bulletin\r\n";
QFile *file = new QFile("E:/Mishana!/1.png");
file->open(QIODevice::ReadOnly);
// add image
data += "--" + boundary + "\r\n";
data += "Content-Disposition: form-data; name=\"up[]\"; filename=\"1.png\"\r\n";
data += "Content-Type: image/jpeg\r\n\r\n";
data += file->readAll() + "\r\n";
data += "--" + boundary + "--\r\n";
m_jar->setCookiesFromUrl(m_cookies, SEC_REQ_ADD);
m_setting.WriteAddReq2(m_request, data.length());
m_reply = m_manager->post(m_request, data);
QObject::connect(m_reply, &QNetworkReply::finished, &m_loop, &QEventLoop::quit);
m_loop.exec();
// №3