Sexual harassment is about exerting power on the victim so the "perp" gets some kind of advantage. The case of the accidental emailing of one's own erotic photographs does not constitute sexual harassment.
I respectfully disagree, though this may vary from state to state. Unwanted sexual contact, references, or images may constitute sexual harrassment; it seems clear that these are unwanted sexual images. OP should at least talk with HR or campus lawyer, and get it on record ASAP that he or she did nothing to encourage this kind of email.
As far as the claim of the student that the instructor opened a student folder they were not supposed to, any member of the university's IT technical staff can testify that this is (a) not technically possible (b) if possible it was the student's responsibility to safeguard the content let alone not have such content on university computers.
Folks, it was an accident. Lets not try to make it more than it is. We already live in a litigious society.
I can agree that we don't need more litigation, but the OP needs a good defense, and that may be a good offense in this case. I'm also not thinking of litigation, but student conduct code violation, which is not a criminal matter (usually).