Use a directory called "iconset" in your theme directory for the files icons (instead of wp-includes/images/crystal)
You have to provide icons for the different types of files. You can provide them as PNG file.
The original icons have following dimension: 46px width and 60px height. It might be important to keep this dimension in order to preserve the general layout of WordPress interfaces.
WordPress manage 9 different types of files :
You have to provide one icon for each type of file.