QNetworkReply определить тип принимаемого сообщения.

QNetworkAccessManager *manager = new QNetworkAccessManager(this);
QNetworkRequest request;
connect( &manager, SIGNAL(finished(QNetworkReply *)), this, SLOT(quit()) );

request.setUrl( QUrl("http://www.my_site.ru/") );
QNetworkReply *reply = manager.get(request);
connect(reply,SIGNAL(finished()),this,SLOT(slot_finished()));

void slot_finished()
{
qDebug() << "content=" <<>header( QNetworkRequest::ContentTypeHeader);
}