Well, two things. First, if it's a supplemental application (e.g., "which tracks you're interested in," etc.) they're not actually allowed to ask for that anymore, so you should be good. The only supplemental materials allowed this year are assessment reports or treatment documents (e.g., treatment plan, case summary, etc.)
If you forgot to include one of those, well, I'm not sure... I'd try emailing the site directly, saying that you noticed your error, and ask if they'd accept the supplemental information as an attachment. From what I understand, sites are technically not allowed to do this, but you might find that some of them are willing to accept it.
For what it's worth, I've found some sites that had previous years' materials up and had actually changed some parts of their internship program... resulting in one of my cover letters talking about opportunities that might not even be available. ::shrugs:: I'm SOL on that one. 🙂. Nothing I can do about it now; just hoping it doesn't count against me too much.