docs: def dump_file_by_path(self, path, **kwargs):
          """
          Returns the concrete content for a file by path.
  
          :param path: file path as string
          :param kwargs: passed to state.solver.eval
          :return: file contents as string
          """
          file = self.state.fs.get(path)
          if file is None:
              return None
          return file.concretize(**kwargs)