docs: def file_exists(self, subdir, prefix, suffix):
          """Returns true if the resource file exists, else False.
  
          Positional arguments:
          subdir -- sub directory name under the resource's main directory (e.g. css or js, or an empty string if the
              resource's directory structure is flat).
          prefix -- file name without the file extension.
          suffix -- file extension (if self.minify = True, includes .min before the extension).
          """
          real_path = os.path.join(self.STATIC_DIR, self.DIR, subdir, prefix + suffix)
          return os.path.exists(real_path)