Might be caused by your web server not allowing specific file types to be downloaded.
Log onto your server and open up Internet Information Services then go to the Default Website and click on the Mime Types icon. Look for entries for the xlsx and docx file extensions, if they don't exist you can add them. I don't know what the mime types for either of those extensions are off the top of my head but you can google it. It's probably something like application\excel or application\word
That might be the problem, might not.