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).

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)