我在Bootstrap 3中有一个modal，我想从用户那里得到一个文件。我的问题是当用户选择了这个文件，但它加载失败，关闭了modal，然后再次打开它，以前的文件仍然存在。我如何在用onclick打开modal的函数中删除这个文件？

<button class="pointer" onclick="return contractUpload('${ml.getId()}')"><a>upload contract</a></button>

<div class="modal fade" id="contractModal">
        <div class="container">
            <div class="modal-dialog" style="margin-top: 15%">
                <div class="modal-content">
                    <div class="modal-header">
                        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                            <span aria-hidden="true">&times;</span>
                        </button>
                        <h4 class="modal-title">contract modal <span id="mId2"></span></h4>
                    </div>
                    <div class="modal-body">
                        <div class="row">
                            <div class="col-md-6">
                                <div class="form-group">
                                    <label class="control-label">select file :</label>
                                    <label class="btn btn-success">
                                        <input type="file" id="contractFile" accept="application/pdf">
                                        <i class="fa fa-cloud-upload"></i>
                                        <span id="browse">choose :</span>
                                    </label>
                                </div>
                            </div>
                        </div>
                    </div>
                    <div class="modal-footer">
                        <button type="button" class="btn btn-success pull-left" data-dismiss="modal">upload</button>
                    </div>
                </div>
            </div>
        </div>
    </div>

<script>
    function contractUpload(mId) {
        $('#contractModal').modal();
        document.getElementById("mId2").innerHTML = mId;
    }
</script>

我使用了下面的代码，但它不工作：

document.getElementById("contractFile").value = "";