As of yet, we don't know how extra attacks will be implemented. I personally, based on current implementation of Haste Attack and Bonus Action Attacks, think that Extra Attacks will be implemented as extra "action points" which the Fighter will be able to spend on any Actions of their choosing. If my hunch turns out to be correct, this means that the EK's "opportunity loss" from casting an Action-costing spell will be much lower, as they will be able to cast a spell and then use their remaining "action points" to attack, or vice versa.
The extra attack is already partially implemented in the game files. From what I've been able to understand, it goes like this:
1. At level 5, you get a passive called 'Extra Attack'.
2. This passive procs once per turn when you attack something and gives you a hidden status buff that lasts 1 turn.
3. This status buff gives you a new kind of resource called 'ExtraActionPoint' and is removed after you attack something.
It doesn't look like anything uses this 'ExtraActionPoint' yet, but we can make guesses. Either the regular attacks will have their costs altered to "1 Action Point OR 1 Extra Action Point" (not sure if that's possible), or it'll be like with the bonus attacks from GWM, after using a regular attack you'll get a new attack "spell" on your hotbar that uses this new resource.