code: public static String getInvalidFileTypeMessage(final List<String> fileTypes) {
  		if (fileTypes == null) {
  			return null;
  		}
  		return String.format(I18nUtilities.format(null,
  			InternalMessages.DEFAULT_VALIDATION_ERROR_FILE_WRONG_TYPE),
  			StringUtils.join(fileTypes.toArray(new Object[fileTypes.size()]), ","));
  	}
docs: Returns invalid fileTypes error message.
  
  @param fileTypes allowed fileTypes
  @return human readable message