upload问题，不知道哪里错了。

upload.html<html>
<head>
<title>upload</title>
</head>
<body>
<form  action="upload.php" method="post" enctype="multipart/form-date">
<input type="file" name="pic">
<input type="submit" name="sub" value="upload">
</form>
</body>
</html>
upload.php<?php
    echo '<pre>';
    print_r($_POST);
    print_r($_FILES);
    echo '</pre>';

?>
------------------------结果：Array
(
    [pic] => D:\Documents\www\示例图片_01.jpg
    [sub] => upload
)
Array
(
)问，为什么$_FILES是个空数组啊？