It was not without a few glitches, but until now the working recipe was to create a folder within the activity's folder and create a relative link pointing to its content something like this:
Now there's a problem as many browsers will not display iframes that do not point to SSL files.
In other words my link should now be complete and not relative anymore: from "https://" down to the very resource location.
My question is: how do I obtain this location since the file is within the activity's folder?
Many thanks for help and suggestions.