Here, read about it. To get into Extras, an app has to be optified, so it should be optified by the dev before promoting it to Testing. That's what Testing is for, though, to verify that--among other things.

And that's enough talk about Extras-Testing...

I made the final test myself and promoted PrBoom: Doom is now in Fremantle Extras!

Many thanks to those who participated in testing.