我一直收到这个奇怪的错误，我就是想不通

这是我当前的代码：

function addBackground() {
   var penguin_id = getPenguinID();
   var file = $('#image_form')[0].files[0];
   console.log('Image Getting Uploaded');
   var formData = new FormData();
   formData.append('file', file);
    $.ajax({
        url: "custBG.php",
        type: "POST",
        data: formData,
        cache: false,
        contentType: false,
        processData: false,
        mimeType: 'multipart/form-data'
    }).done(function(data) {
        if (data.status == 0) {
                console.log('Image Has Been Uploaded');
                addBGItem(70000 + penguin_id);
        } else {
            console.log(data.message);
        }
    });
}

这是表格

<form id="image_form">
<div class="left">
<img id="img-uploaded" src="http://dummyimage.com/210x210/dbdbdb/7e7f80.png" alt="your image" />
</div>
<div class="right">
<input type="file" name="imageToUpload" accept=".png,.jpeg,.jpg,.gif" onchange="validate_image()">
<span class="btn btn-large btn-alpha" onclick="addBackground()">Upload Image</span>
</span>
</div>
</form>

我不确定发生了什么，但有人可以帮我解决这个问题吗？