问如何使用ajax提交文件上传表单？

EN

<iframe name="my_iframe" id="my_iframe" style="display: none;"></iframe>

<form action="/next_step.php" method="post" target="my_iframe" entype="multipart/form-data">
    <input type="file" name="file_upload" />
    <input type="button" id="upload_btn" value="Upload file" />

    <label for="name">Name</label>
    <input type="text" name="name" id="name" />

    <input type="submit" value="Next step" />
</form>

<script type="text/javascript">
    var btn = document.getElementById('upload_btn');
    btn.onclick = function(){
        var form_elem = document.forms[0];
        // direct file uploads through the hidden iframe
        form_elem.action = '/test_file.php';
        form_elem.target = 'my_iframe';

        // submit the form
        form_elem.submit();
    };
</script>