I did some digging around and I noticed that in functions.php the mediabox code seems inconsistent with the shortcode in the dummy content and documentation.
I found out what caused this problem: the dummy content for the shortcode page contains mediabox shortcode that does not work, probably this is an older version of the shortcode for media boxes. The attributes have changed.
The info in the documentation is correct and works.
To prevent this confusion you might want to update the dummy content.