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The best way I've found so far is to use the PythonExpression subsitution. Here I append the filetype suffix to a string.

world_name = LaunchConfiguration('world')
world_filename = PythonExpression(expression=["'", world_name, "'", " + '.world'"])

Fellfalla's solution is good too and I'd like to see it get PRed into the main repository.

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