Yeah, DLing .APKs has been known to end up with a .ZIP interface - it could be IE (doubt it b/c it has worked for me in the past on all browsers) or it could be your company firewall scanning the file and changing the extension b/c malware tends to masquerade as other files to slip past corporate firewalls and anti-malware defense systems....so the firewall or anti-malware defense system reads the file header, determines its a .ZIP created by 7-zip, and tried to be helpful by renaming the file back to the .ZIP extension.