add_filter('upload_mimes', 'custom_upload_types');

function custom_upload_types ( $mimes=array() ) {
	
	// add to the array your extension => mime_type -- google around for the mime type
	$mimes['rar'] = 'application/x-rar-compressed';
	
	// you could also use this to remove certain default types
	// unset( $mimes['ext'] );
	
	// check the wp_check_filetype function in wp-inclues/functions.php to see the default list!
	
	// now we be sure to return the newly modified list
	return $mimes;
	
}