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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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))