$boundary = uniqid( "");
$headers = "Content-type: multipart/mixed; boundary= $boundary\r\n";
$headers .= "From:$from\r\n";
//确定上传文件的MIME类型
if($_FILES['upload_file']['type'])
$mimeType = $_FILES['upload_file']['type'];
else
$mimeType ="application/unknown";
//文件名
$fileName = $_FILES['upload_file']['name'];

(编辑:aniston)