def get_file_stream(self):
        """
        Return a file stream instance of the upload.
        """
        if self.file_stream:
            self.file_stream.seek(0)
            return self.file_stream
        elif os.path.isfile(self.file_path):
            return open(self.file_path, 'rb')
        else:
            raise ValueError("invalid file {}".format(self.file_path))
Return a file stream instance of the upload.