This is a really good first card idea, Multitallented! I'm excited to see what else you can think of! Also, welcome to the Dominion forums!
Also, ignore tristan. He seems to think that caring about the rules is the same as nitpicking about them. (You don't have to remove the Attack type, though; it still works, it's just somewhat confusing.)
You obviously don't get that the ideal causality chain is from mechanics to rules, no the other way around as rule-lawyers like you wrongly assume. No game designer does it the other way around. You have an idea, you implement it mechanically and only then do you write a rule which formalizes this very mechanic.
If you want the other player to be able to play Reactions to this novel form of Attack, a Reaction-Attack (read the friggin' title of this thread, that's the key point of the card), it should be a Reaction-Attack. If you don't want them to be able to React you don't call it an Attack. In the former case, Reactions can obviously only be played if the Reaction Attack occurs, not if the card is applied "peacefully". That does not match the rules but it is common sensical and most likely how my gaming group would play intuitively with this card.
Rules do exist to clarify how a game is played, not to make it something superrigid which can never be modified.
You certainly do not limit your design ideas to what already exists and has been formalized via the rules. As I already said, nobody does this, especially not DXV. He changes or amends the rules when he implements something new. This is precisely what Multitalented should do, only that there is no need to write an actual rule amendment as he can explain this in 10sec to anybody he plays with.
You just don't get it.
I know very well that Reactions can only be revealed if somebody plays an Attack card. But you don't seem to get that mechanisms come first and rules later.
Externalize is something NEW, a Reaction Attack. The point of the card, as I read it, is that playing the card doesn't count as Attack whereas revealing it as a Reaction does and that in this case other players could reveal Reactions.
Now my reading could be wrong and this could very well not be what Multitalented is after. If he isn't he should indeed just remove Attack as the play effect of the card is not aggressive/interactive at all.
But if it is what he wants then you need a rule amendment that clarifies that Reactions cannot be revealed when somebody plays a Reaction Attack, only when he reveals a Reaction Attack. Of course it would also make sense to give it a new type, Aggressive Reaction or Attack/Reaction or whatever without the dash that seperates the Attack and Reaction type.
Furthermore you have to keep in mind that in principle (not in the case of this card though) this new type includes the potential for infinite loops so one has to be careful.
Again, all I am saying is that a fan card designer should first think about what he wants to achieve mechanically and only then formalize it which includes potential rule amendments. Nothing wrong with doing fan cards that operate within existing parameters but a lot wrong with telling folks who do fan cards that they should never ever come up with something that implies rule changes.
Addendum: Just read Plague Doctor and this is obviously a far cleaner way to implement the mechanism than what I thought up. Great work, Multitalented. And, on a totally egocentric note in this pointless debate with the rulelawyer fraction, kinda entertaining to see that I was initially right about you wanted the card to actually do.