code: public String getMimeType(final String fileId) {
  		FileWidgetUpload fileWidget = getFile(fileId);
  		if (fileWidget != null) {
  			return FileUtil.getFileMimeType(fileWidget.getFile());
  		}
  		return null;
  	}
docs: Retrieves an mime type of the uploaded file's contents.
  This is not the content type passed by the browser.
  
  @param fileId the file id
  @return an file's mime type, or null if no file has been uploaded