It's an encrypted firmware image. You're not supposed to open the file, just right-click on the link to the image and download it.
Installation instructions are in the release notes: http://www.readynas.com/RAIDiator_4_1_8_Notes
If you only need to install it on one machine it would be easier to simply update via System > Update > Remote. That way the NAS itself will download the image off the web.