Actually a text file will simply open and display in most browsers, it's only text - just like HTML is only text.

But you are right about more complex files types, the browser will more often than not simply download the file to the local computer, the local PC will then handle the file based on it's type and associated application which opens it..

There are no add-ons for Web Designer to help with local file handling.
There are server-side scripts that can force a download of file types rather than have them open (JPG, PDF and so on). But again, this is well outside the designed function of Xara software.