def METHOD_NAME(self, digest, content_type, filename):
    """Serve the file with the given digest.
    This will just add the headers required to trigger
    FileServerMiddleware, which will do the real work.
    digest (str): the digest of the file that has to be served.
    content_type (str): the MIME type the file should be served as.
    filename (str): the name the file should be served as.
    """
    self.set_header(FileServerMiddleware.DIGEST_HEADER, digest)
    self.set_header(FileServerMiddleware.FILENAME_HEADER, filename)
    self.set_header("Content-Type", content_type)
    self.finish()