<?php
$attachments = array( WP_CONTENT_DIR . '/uploads/file_to_attach.zip' );
$headers[] = 'From: My Name <myname@example.com>';
$headers[] = "Content-type: text/html" ;
wp_mail('test@example.org', 'subject', 'message', $headers, $attachments );
?>
ref: http://codex.wordpress.org/Function_Reference/wp_mail