Since the upgrade to Wicket 1.4.17 the PortalSiteManager export feature only produces empty (zero size) download zip files.
This turns out to be caused by it now requiring explicit setting of the download file length, or leave it at -1.
The current implementation, which still worked (and works!) with previous used Wicket 1.4.15, uses default and hard coded file length 0, which since the upgrade seems to be taken quite literal.
Fixing by determining the actual file length at runtime and returning that (which potentially also improves performance, although in this case probably negligible).