PHP 判断isset($_FILES)

<form action="2.php" method="post" enctype='multipart/form-data'>

    <input name="upgradefile" id="upgradefile" type="file" size="60">

    <input type="submit" name="">

</form>

不管是否选择了文件, 2.php中判断isset($_FILES)都为真, 但是: error=4表示无文件上传

 $_FILES = Array ( [upgradefile] => Array ( [name] => [type] => [tmp_name] => [error] => 4 [size] => 0 )   )