XPS10 - connecting to PC via USB not working?

Hi,

I'm owner of Dell XPS10 tablet. Running fully updated Windows RT 8.0. According to manual I should be able to connect tablet to my PC as some kind of mass storage, using microUSB-USB cable. But device is only charging, nothing happens both in RT and desktop (Win8) device manager.

USB port is working OK - if I attach USB OTG cable, I can connect USB devices to tablet without problem. Any clules?