previewer: fix folders identifiers in zip archive
previewer: fix folders identifiers in zip archive

Reported-by: Konstantinos Kostis <237b836d1007d10b89d97a8c1749c99bbd9efc26@cern.ch>
Signed-off-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

inveniosoftware/invenio-previewer,inveniosoftware/invenio-previewer,inveniosoftware/invenio-previewer,inveniosoftware/invenio-previewer