What is the specific use case to use the IFeedback?
If you call a Java action from a microflow that results in downloading a file you could create a file object in the java action return that to the calling microflow and then use the regular activities in a microflow to trigger a file download.
However, if you trigger this Java code as part of a request handler then you need to provide the file via the HTTP response. For this there are many code examples available. But none of them will be using the IFeedback interface.