如何用PHP获取通过BackgroundUploader上传的文件

Question

我按照这个页面的示例：http://msdn.microsoft.com/zh-cn/library/windows/apps/hh700372.aspx

实现了选择图片并上传的操作，但是在PHP中怎么获取不到文件？$_FILES 是空的。

            function UploadOp() {
                var upload = null;
                var promise = null;

                this.start = function (uriString, file) {
                    try {

                        var uri = new Windows.Foundation.Uri(uriString);
                        var uploader = new Windows.Networking.BackgroundTransfer.BackgroundUploader();

                        // Set a header, so the server can save the file (this is specific to the sample server).
                        uploader.setRequestHeader("Filename", file.name);

                        // Create a new upload operation.
                        upload = uploader.createUpload(uri, file);

                        // Start the upload and persist the promise to be able to cancel the upload.
                        promise = upload.startAsync().then(complete, error, progress);
                    } catch (err) {
                        $.dialog(err);
                    }
                };
                // On application activation, reassign callbacks for a upload
                // operation persisted from previous application state.
                this.load = function (loadedUpload) {
                    try {
                        upload = loadedUpload;
                        promise = upload.attachAsync().then(complete, error, progress);
                    } catch (err) {
                        $.dialog(err);
                    }
                };
            };

            document.getElementById("upload_avatar").addEventListener("click", function () {
                var picker = new Windows.Storage.Pickers.FileOpenPicker();
                picker.fileTypeFilter.replaceAll([".jpg", ".bmp", ".gif", ".png"]);
                picker.pickSingleFileAsync().then(function (file) {
                    if (file) {

                        var upload = new UploadOp();
                        upload.start("demo.php", file);
                    };
                }, function (error) {
                    //error
                });
            }, false);

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