With this patch it's possible now to explicitly set the ZBlk format of a
single ZBigFile. Before this patch it was only possible to set the ZBlk
format globally, for all ZBigFile. This new feature is useful if we have a
project where we would like to have mixed ZBlk formats, e.g. some
ZBigFile with ZBlk0 and some ZBigFile with ZBlk1.