A little enhancement for multiple file upload

Various javascript solutions for multiple file upload on the client side upload all files on one and the same key in the form. Therefore, what I did was just to change the map from <String, UploadedFile> to <String, List<UploadedFile>> in MultipartDecoder service. And driven by the least number of changes idea I added a new component called MultiUpload instead of changing the existing Upload. The new component just binds List<UploadedFile> instead of a single UploadedFile.