Last night I got some pictures one of my friend sent me as a zip file as email attachment, I wanted to open and see those pictures on my iphone but with the when I downloaded the zip file using the native mail app on iphone, it downloaded but did not open as it was a zip file but could have opened easily if it was an image or some document.
In order to open and view the zip file contents, you will need to install a paid app($2.99) called Air Sharing from apple app store to open zip file and to open .rar and . zip file you can install USB Disk which is a free app on apple app store, both which will allow you to open and view zip and rar files in email attachments on your iphone.
Note: You can also try some other similar apps on app store to open zip and rar files.
Once you have install the air sharing app, open the mail containing zip file for which you will need to tap and hold the zip files and rar files after downloading as shown in the image below.
Once tap and hold over attachments after downloading them, you will see the option to open them in air sharing and usb disk or any other similar apps like these as shown in the image below.