We have a few advanced techniques that inspect the file data and ensure that the file is valid. So even if you rename your encrypted backup to to My_Home_Video.avi, it wouldn't pass our check and would fail to upload.

And what about lossless image formats like .tiff or .bmp? These allow you to basically store any data in an image, and although the photo looks just like noise it's a completely valid image file with minimal overhead.

I do know some people that use that for backups on picasa and flickr, which have unlimited storage and 1TB respectively.