Enable Recent Comments widget to display comments on attachment posts

you can use the widget_comments_args filter to modify the default args of the recent comments widget:

function wpse80087_widget_comments_args( $args )
{
    $args = array( 'number' => 5, 'post_type' => 'attachment', 'status' => 'approve', 'post_status' => 'inherit' );
    return $args;
}
add_filter( 'widget_comments_args', 'wpse80087_widget_comments_args', 10, 1 );