Hi
I am not sure if this is as designed. but i can see that in dialog box there is not button for uploading.
Workaround is on the same dialog box click the maximize icon on top right, it will show you the "/mnt/overlay/wcm/core/content/sites/properties.html?item=PATH TO MY PAGE" window.
See
I would request you to please create a daycare ticket for this.
~kautuk
Kautuk Sahni