CAST SPELL ACTION
1. Declare spell
2. Pay cost
3. Counter Spell
4. Resolve Spell
ATTACK ACTION
1. Declare attack
2. Pay cost
3. Roll to miss
4. Avoid attack
5. Roll Dice
6. Damage & Effects
7. Additional Strikes
8. Damage Barrier
9. Counterstrike
10. Attack Ends
If i reveal Maim Wings in the cast spell action between step 1 and 2 or 2 and 3 or 3 and 4 that would before the spell resolves and becomes an attack action.
That makes a certain gamey sense. However, on reading more closely, allow me to counter with this:
Codex: "An attack action is a creature's action (quick or full) used to make an attack"
Rules (pg 23): When you cast an attack spell, it is both a spell and an attack."
So, by way of example, a wizard uses his quick action to cast a lightning bolt attack spell.
Since the attack spell is both a spell and an attack (as per rulebook), he is therefore using his quick action to make an attack (as per the Codex). So as soon as he casts the Lightning Bolt, we are in an Attack Action.
As you cannot gain or lose flying during an Attack Action, revealing Maim Wings during the Cast Spell Action steps of the Lightning Bolt's resolution will not avail - the attack action has started
I think this makes more sense in terms of consistency and faireness ("What do you mean I can drop Felella behind a wall to avoid a fireball, but not to avoid a catapult stone? How does that work?" and so on.)
However, this game being what it is, there is quite a high likelihood that the writers have inadvertently used "Attack Action" in two or more different ways.
Does the key rule about flying (pg15) refer to "Attack Action" as per the Codex' definition (as I think) or to the formal set of steps listed under "Attack Action" on the back of the rulebook (as you think)? It is for better minds than mine to decide.
(Interestingly, I note that the Attack Action table on the back of the rulebook is not replicated inside the rulebook. While the same table appears on page 23, it has a different title ("Making an Attack"). Make of that what you will.)