Just an interesting 'mistake' made by google, if one adds filetype:xls for example, one is not expecting to see a "Image results for your query:" box like with the following query: http://www.google.com/search?hl=en&q=filetype%3Axls+country and if you try to search for http://www.google.com/search?hl=en&q=filetype%3Agif+country&btnG=Search it becomes even worse, because the second file is a pdf, and only the url ends with gif, but google knows its a pdf, because they suggest the HTML version.

The xls query shows images because the path to those images have '.xls' in it.