this used to be common with txt files and IE's terrible practice of sniffing content. It would see a txt file that contained html and display the html instead, it could then pull in a secret silverlight file that was mascarading as a docx file as they are both simply zip files. Even more amusingly silverlight and docx contents don't clash so it could still be a valid docx file if you opened it, and the txt file would look like txt even though it was really rendering html with a hidden silverlight app.