Attack.cfg is what you would use when you are announcing incoming enemies.

The default settings are (you can change them if you want in incoming.cfg):

F1 Incoming SCOUT
F2 Incoming SNIPER
F3 Incoming SOLDIER
F4 Incoming DEMOMAN
F5 Incoming MEDIC
F6 Incoming HWGUY
F7 Incoming PYRO
F8 Incoming SPY
F9 Incoming ENGINEER
F10 Incoming UNITS (2-3)
F11 Incoming UNITS (4-30)