def METHOD_NAME(self, path, callback):
    abs_path = self.path_on_filesystem(path)
    def file_exists(resource_available):
        if not resource_available:
            callback(None)
        else:
            with open(self.path_on_filesystem(path), "rb") as f:
                callback(f.read())
    self.exists(None, file_exists, path_on_filesystem=abs_path)