新手求助.php上传总是 invalid file

<html>
<head>
<title>upload_test</title>
</head>
<body>
<form action="upload_file.php" method="post" enctype="multipart/form-data">
<label for="file">Filename:</label>
<input type="file" name="file" id="file" />
<br/>
<input type="submit" name="submit" value="Submit" />
</form>
</body>
</html>------------------------------------------------------
upload_file.php
------------------------------------------------------
<?php
    if ( (($_FILES["file"]["type"] == "image/gif")||($_FILES["file"]["type"] == "image/jpeg")||($_FILES["file"]["type"] == "image/pjpeg")) && ($_FILES["file"]["size"] <20000) ){
            if ($_FILES["file"]["error"] > 0){
                echo "Return Code: ".$_FILES["file"]["error"]."<br/>";
                }else{
                    echo "Upload: ".$_FILES["file"]["name"]."<br/>";
                    echo "Type: ".$_FILES["file"]["type"]."<br/>";
                    echo "Size: ".($_FILES["file"]["size"]/1024)."<br/>";
                    echo "Temp file: ".$_FILES["file"]["tmp_name"]."<br/>";
                    
                    if (file_exists("upload/".$_FILES["file"]["name"])){
                        echo $_FILES["file"]["name"]."already exists.";
                        }else{
                            move_uploaded_file($_FILES["file"]["tmp_name"],"upload/".$_FILES["file"]["name"]);
                            echo "Stored in : "."upload/".$_FILES["file"]["name"];
                            }
                    }
        }else{
            echo "Invalid file";
            }
?>
--------------------------------------------------------------------------------------
W3School教程中的文件上传，我复制这两个文件，一在浏览器下上传文件，就只返回 invalid file