public void Unknown_extension()
{
Assert.Throws <Exception>(() => AllFiles.GetPathFor("txt"));
Assert.False(AllFiles.TryGetPathFor("txt", out var result));
Assert.Null(result);
Assert.False(AllFiles.TryGetPathFor(".txt", out result));
Assert.Null(result);
Assert.False(AllFiles.TryCreateFile("foo.txt"));
Assert.Null(result);
Assert.Throws <Exception>(() => AllFiles.GetPathFor(".txt"));
Assert.Throws <Exception>(() => AllFiles.CreateFile("foo.txt"));
}