In terms of file formats, right now we don't have a good idea what will be covered, so all these are just guesses. But it's pretty safe to assume that any .ATR file will work, as well as .XEX files.
I'd expect the vast majority of .CAR/.ROM files to be supported, for example 5200, standard 8K, 16K and XEGS carts, but because the 8-bit computers had so many different bank selection schemes in ROM carts, there will probably be some carts that either aren't supported, or not properly recognized. That's something that's easy enough to fix with firmware updates, but I wouldn't expect 100% compatibility at initial ship.