This is because the internet browser could not automatically determine the format of your full-size image from the image extension and the file header.
The easiest way to fix this would be changing the fullsized images to jpg or png (etc) format.
Otherwise you have to set up the MIME type of the images in the content header to tell the browser what type of image it is. If the image is not in a standard format that internet explorer understands, then you need to supply custom plug-in's (can set up via tools/folder-options/file-type).