Even though it has all data available, if a network reply has not been read from when finished() is emitted, it claims to be sequential, and does not allow seeking, even though all data can be read from it (with readAll()). This causes problems with other parts of Qt, such as image readers, which only partially work (if at all) for sequential devices. Basically, it forces the user to write:

QByteArray all = reply->readAll();
QBuffer buff(&all);
buff.open(QIODevice::ReadOnly);
QImageReader reader(&buff);

rather than:

QImageReader reader(reply);

presumably this adds yet another buffer copy to process network data.