ng-file-upload multiple files to spring rest service

I am using ng-file-upload to upload files to spring rest service. Single file can be uploaded successfully, but failed when multiple files.
The code I use is below:
JavaScript:

                    $scope.uploadFiles = function (files) {
                    $scope.files = files;
                    if (files && files.length) {
                        Upload.upload({
                            url: 'http://localhost:8099/test/upload2',
                            data: {files: files}
                        }).then(function (response) {
                            $timeout(function () {
                                $scope.result = response.data;
                            });
                        }, function (response) {
                            if (response.status > 0) {
                                $scope.errorMsg = response.status + ': ' + response.data;
                            }
                        }, function (evt) {
                            $scope.progress = Math.min(100, parseInt(100.0 * evt.loaded / evt.total));
                        });
                    }
                };

Java:

@ResponseStatus(HttpStatus.OK)
@RequestMapping(value = "/upload2")
public void upload2(@RequestParam("files") MultipartFile[] files) throws IOException {
}

But the files array is empty, anything wrong for multiple files?

Update1:
I can see the request in network tab as below:

------WebKitFormBoundarywCLpwRaJRcmfPiBl
Content-Disposition: form-data; name="username"

qqq
------WebKitFormBoundarywCLpwRaJRcmfPiBl
Content-Disposition: form-data; name="files[0]"; filename="eagle.jpg"
Content-Type: image/jpeg


------WebKitFormBoundarywCLpwRaJRcmfPiBl
Content-Disposition: form-data; name="files[1]"; filename="eclipse.epf"
Content-Type: application/octet-stream


------WebKitFormBoundarywCLpwRaJRcmfPiBl--