From looking at your other thread it seems you have already figured out this part 
You could also use an even shorter variant of this:
{{$json["filename"] || "default"}}
This uses JavaScript’s short-circuiting logic for the OR operator (||). If $json["filename"] already returns a truthy value, the part after the || isn’t evaluated. But if $json["filename"] returns something falsy (for example because no filename was provided by the previous node), "default" is returned.