Problem: You've just downloaded your .zip file. It looks odd. The file extension is '.zip.cpgz' either before or after trying to extract the contents. Any attempt to extract the file sends you in a loop.
Cause: This happens more regularly with files that are greater than 1GB in size. However, it has been known to happen with smaller files. It has to do with a bug in the built-in unzipping tool on MacOS.
Fix: Download and install 'The Unarchiver' from the App Store. It kicks ass!