At the moment it seems it is not possible to stream back to the browser a character or binary content (as response attachment) after using an action prompt (modal window).
If the action doesn't require filling parameters then firing it will execute a normal (non-Ajax) request and streaming content (e.g. PDF, DOCX) is not a problem.
But if there are parameters to be provided then Isis Wicket viewer open a modal window and its OK/Submit action is executed in Ajax request. To be able to stream any Lob content back to the client we need to do something like https://cwiki.apache.org/confluence/display/WICKET/AJAX+update+and+file+download+in+one+blow