I had a very brief "affair" (if that's even what you can call it)with my ex.
Good heavens, what else would you call it. Don't try to minimize what you did by trying to describe it as anything other than what it was. You had an affair. You chose to cheat on your husband.
So, bascially an ex boyfriend and you got together after the two of you were married to others and had sex 1 time. His wife found out and now claims she has video/pics of what? Did she say? Since you say you are unaware of any video/pics that your ex would have taken, maybe ask him if he did secretly take any. Hopefully you were not dumb enough to send him nude photos of yourself while you two were seeing each other.
My guess, assuming your ex has enough decency not to have secretly video taped you without your knowledge, is that she is bluffing. If you are sure of that, then do nothing. Of course, if you are wrong...
As for your husband, I agree with a previous poster. Ask yourself. Is it better if he finds out from me or from another source. Because if you call this woman's bluff and she doesn't have pics/video, she may well contact your husband.
I would wish you well, but I have more sympathy for your husband than I do for you. Whatever happens, you will have deserved it. Actions have consequences and you may be about to face them. Hope it all works out well for your husband.