My mistake, not espionage, treason. Which is worse, in terms of the sentences available.
You might think that a Swedish citizen can't be charged with treason to America. But as I understand it, the guy who passed the files to him is in jail awaiting trial for treason (and being seriously abused there, but that's to be expected.) If there is a prima facie case that Assange invited him to do so, rather than simply being given the disc out of the blue, he can be charged as an accessory.