Add them as files of any type you want and create a design for correct MIME type. E.g. for iCal:
<MySource_PRINT id_name=“global” var=“content_type” content_type=“text/Calendar” />
<MySource_PRINT id_name=“global” var=“content_attachment” extension=“ics” />
<MySource_area id_name=“body” design_area=“body” />
If you just need different output for different kinds files stored as File assets, you’ll probably need to use conditional keywords in the File type format, for example: