upload problem:$_FILES always is empty!

<?php
print '
	<form method="POST" enctype="multipart/form-data" action="[~[*id*]~]">
		<input type="file"   name="upload" />
		<input type="submit" value="upload" name="submit" />
	</form>
';

if(isset($_POST['submit'])){
	if(empty($_FILES))
		print '<b>empty</b>'.$_FILES["upload"]["error"];
}
?>

[!YourSnippet!]