WordPress at a glance
filter is not described

icon_dirs filter-hook . WP 2.5.0

Filters the list of icon directory URIs.

Usage

add_filter( 'icon_dirs', 'filter_function_name_2989' );
function filter_function_name_2989( $uris ){
	// filter...

	return $uris;
}
$uris(array)
List of icon directory URIs.

Where the hook is called

wp_mime_type_icon()
wp-includes/post.php 5599
$dirs = apply_filters( 'icon_dirs', array( $icon_dir => $icon_dir_uri ) );

Where the hook is used (in WP core)

Does not used.