Return a concretization of the contents of the file, as a flat bytestring.

def concretize(self, **kwargs):
        """
        Return a concretization of the contents of the file, as a flat bytestring.
        """
        size = self.state.solver.min(self._size, **kwargs)
        data = self.load(0, size)

        kwargs['cast_to'] = kwargs.get('cast_to', bytes)
        kwargs['extra_constraints'] = tuple(kwargs.get('extra_constraints', ())) + (self._size == size,)
        return self.state.solver.eval(data, **kwargs)