为什么我在用jQuery上传AJAX文件时遇到错误?

我正在使用HTML5的FormData和jQuery开发一个多文件上传器。我有以下代码:

$(function() {
    $('#file').bind("change", function() {
        var formData = new FormData();
        //loop for add $_FILES["upload"+i] to formData
        for (var i = 0, len = document.getElementById('file').files.length; i < len; i++) {
            formData.append("file" + i, document.getElementById('file').files[i]);
        }
        //send formData to server-side
        $.ajax({
            url : "file-upload.php",
            type : 'post',
            data : formData,
            dataType : 'json',
            async : true,
            processData: false,  // tell jQuery not to process the data
            contentType: false,   // tell jQuery not to set contentType
            error : function(request) {
                alert('error!!');
                console.log(request.responseText);
            },
            success : function(json) {
                alert('success!!');
                $('#upload-result')[0].append(json);
            }
        });
    });
});

我可以看到我的file-upload.php工作正常，因为文件已上传!但是为什么我的jQuery击中错误函数，而不是成功函数?我得到一个错误的警告。

在控制台窗口中，我看到了PHP echo调用的结果!如成功函数所示，我想将其附加到#upload-result。 PHP代码

foreach($_FILES as $index => $file)
{
    $fileName     = $file['name'];
    // echo '[ file name: ' . $fileName . ' - ';
    $fileTempName = $file['tmp_name'];
    // echo 'file temp name: ' . $fileTempName . ' ]';
    if(!empty($file['error'][$index]))
    {
        // some error occured with the file in index $index
        // yield an error here
        return false; // return false also immediately perhaps??
    }
    // check whether it's not empty, and whether it indeed is an uploaded file
    if(!empty($fileTempName) && is_uploaded_file($fileTempName))
    {
        // the path to the actual uploaded file is in $_FILES[ 'file' ][ 'tmp_name' ][ $index ]
        // do something with it:
        move_uploaded_file($fileTempName, "uploads/" . $fileName);
        echo json_encode('<p>Click <a href="'. 'uploads/' . $fileName . '">here</a> to download file!</p>');
    }
}

dataType: 'html',