Well it ist not such a big mystery: it’s all about CSS3 pattern matching selectors and pseudo-elements. For example:

a[href$=".pdf"] {

      [your css here]

}

modifies any link to a pdf-file according to your css. In the case of File Icons it obviousely is a background image and some padding.

Now here I’ll do a wishlist for the version 1.1 of this plugin (you can add through the comments, if you want):

• options page featuring an interface for adding patterns to match
• adding selectors to these patterns (for example to match only if within the .content - div)
• option to have an icon for outgoing links
• option for posts to turn of the icons