You would do it in two parts. First upload the file to the server, check it's safe and virus free etc. Next build your email and inlude the attached file in one of the email headers e.g. To do a MS word file you might have something like:
$message .= "Content-Type: application/msword; name=\"my attachment\"\n";
$message .= "Content-Transfer-Encoding: base64\n";
$message .= "Content-Disposition: attachment; filename=\"$attachement\"\n\n";
Where $attachement is the variable containing the file you uploaded in the first part.
More info can be found by RTFM at php.net. Look under mail and mail().
http://cn.php.net/manual/en/ref.mail.php
http://cn.php.net/manual/en/function.mail.php