code: public File getFile(String fileName) throws IOException {
      validatePathname(fileName);
  
      String pathname = storageDir + separator + fileName;
  
      File file = new File(pathname);
      if (!file.exists()) {
        throw new FileNotFoundException(pathname);
      }
      if (!file.isFile()) {
        throw new IOException('\'' + pathname + '\'' + " is not a file");
      }
      return file;
    }
docs: Returns a {@link File} for the given filename in the store.
  
  @throws FileNotFoundException if no file with the given filename exists
  @throws IOException if the given filename does not refer to a file