Safari on Mac OS X has an annoying convenience feature when it comes to downloads. It will automatically open files that it believes to be safe. While I was trying to submit a malicious attachment to an antivirus vendor, I found this to be a concerning, insecure practice. Fortunately, there is a way to disable it.
Launch Safari and from the menu navigate to:
Under the General tab, you will find a checkbox at the bottom that says ‘Open “safe” files after downloading’ “Safe” files include movies, pictures, sounds, PDF and text documents, and archives’.
Uncheck that checkbox and close the Preferences. The next time you download a .zip attachment, it will remain in the compressed archive format instead of being automatically extracted.