What is your fear exactly? That she might stumble upon it accidentally (faked, manipulated search results from bogus servers), or that she might directly seek it out because she's curious?
If it's the second, it would be much better to talk to her about these things instead of trying to censor her. You see, there's nothing inappropriate about sex. Sure, a lot of today's porn is over the top, but again, talk to her about what porn is, why it's there and how in many cases it doesn't accurately represent what sex is about.
But if she's too young for such a talk, then you simply don't let her at the computer without supervision. You won't be able to censor everything anyway, even if you put in a word filter or something. And porn is not limited to aMule, she's bound to find it somewhere if she's at the computer unsupervised. Plus, it's not like a naked body will permanently damage her or something.
That said, limiting to specific type (say, mp3s) will help, so you do have a valid feature request, but it definitely wont cure your problem.