Often its difficult to figure out which application should you use for the files with unknown or unfamiliar extensions. However, you can quickly do so by using WolframAlpha web service. In order to search for the relevant applications that can open the file, simply head over to Wolfram|Alpha: Computational Intelligence and enter the file extension as shown in this image. I've used extension as " .key " to search for relevant applications.



At the bottom of computed search result, the service shows which applications can open the file. Useful, right?