Since Adobe withdrew Flash support there is a workaround that will allow you to access the Unitiserve via what used to be known as the Browser Interface. This is what used to appear if you typed the IP address into a browser. Instructions here:
I still find this interface useful, but it is not what you want for transferring files onto the US Downloads folder. Not sure why you can’t do this by simple copy and paste.
The Core is a very different design to the US, albeit that the firmware is based on it. Even if you could get hold of the ‘innards’ I don’t think you would get them into a US box. Personally I wouldn’t want to given that the Core lacks a number of useful functions that the US has.