If file to read is over 4GB and bufferSize is big, reading is failed:

qint64 bufferSize = (qint64)4*1024*1024*1024;
file.read(tempArray,bufferSize);

This is because in QFSFileEnginePrivate::nativeRead(char *data, qint64 maxlen), DWORD crops the size:
DWORD bytesToRead = DWORD(maxlen);

Reading is succeeded if the bufferSize is smaller.