docs: 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)