You learn something new everyday. To quote one of my new favorite links
GIF files have their length defined at the start of the file; any bytes after are ignored. ZIP files have a table at the end; anything at the start of the file is ignored. The result is that a file can be both a GIF and a ZIP, just change the extension.
So .gif files are useful after all! … Just not as gif’s.
This picture can be renamed as a .zip, and it will contain the source for DeCss: