There's this extension called TargetAlert that displays an image next to links that point to files with specific extensions. So if there is a link to a .pdf file, it will show the Adobe logo next to that link. Well there's nothing special about that except that when I went to read the comments to that extension, I found one comment that tells you how to do it without any extensions and only by editing your "userContent.css" file, which I found rather neat.

To do it, just add the following code to your userContent.css file:

Nice, thanks!!!