You can use the Reformat Activity. But this will consume another additional AI unit.
I would recommend specifying the Output style in your Content Generation Activity’s System Prompt along with an example Input and Output.
It would still be of a String type, but the format will be maintained as per the one specified in the Prompt.
If your requirement is to have the Output as a file (output.csv, output.txt, …), i don’t think it’s possible, the output will always be a string.
I hope this answers your query