code: public File getFilteredVersion(File input) {
          File out;
          if (!input.getName().endsWith(".css")) {
              out = getOutputFile(input, "css");
          } else {
              out = getOutputFile(input);
          }
  
          if (!out.isFile()) {
              return null;
          }
          return out;
      }
docs: Overrides the parent method to manage the case where the given file is not a `.css`. In that case it should
  strips the extension and find a `.css` file.
  
  @param input the input file
  @return the filtered file (to the mirror of the file in the output directory), {@code null} if not found