Before you check your Gmail attachment you had a way was download or view, but for .zip file or .rar file, you could only download.
The newest feature from Gmail: you can view .zip and .rar file in your Gmail directly now without downloading (but you have to know the Gmail still does not allow you send or receive any email with .exe, .dll, .ocx files included.)
The following is a sample email which includes a .zip file. You can see there is a “View” link on the bottom of content:
When you click the View link, you will be accessed to Google Docs, and the .zip files will be listed on Google Docs, now you know Google Doc Viewers can view .zip and .rar files: