code: public void addFile(String fileName, Integer fileSize, String thumbnailUrl) {
  		addFileNative(getElement(), fileName, fileSize, thumbnailUrl);
  	}
docs: Add file information manually to Dropzone. Added file is not uploaded, it is just shown in the dropzone.
  This feature is useful for displaying e.g. files that already exists on the server.
  
  @param fileName     name of file to add
  @param fileSize     size of file to add
  @param thumbnailUrl thumbnail image for file