Well, firstly, I'd like to point out that it's not so much about the forum's rules..as it is about the real legal obligations of it. The forum simply has strict rules because it doesn't need to be at blame for content not properly owned by the user being linked/posted here. It's a fair rule. Similarly, Knytt Stories Level Archive is the same way about such things if you didn't make it. It's run by J, though, if I remember correctly. Point is, it's not really about what -we- tell you can do and can't do...it's more about the fact it's just downright illegal to distribute things you don't have the rights to distribute, modify or use.
Basically, you are taking a chance by posting it, hosting it, uploading it, or maybe even having used it to begin with, depending on what it is. The forum won't tolerate it, though, and it'll be removed from here. Doesn't matter if it's hosted here, or somewhere else, etc. There are lots of famous levels that have not been allowed to be reposted, take for instance; "Don't eat the mushroom". Tons of game graphics ripped from a game, the music, obviously, etc.
Depending what -exactly- is copyrighted, or not yours to use, you could try to ask for permission, or find something just as good, that you -can- use. (This is actually easier than people think). Please remember that also if something is not claimed- it doesn't mean it's free to use, though. And additionally, you should always have the courtesy to credit everyone's stuff properly. Think of it as a both a huge thank you, and payment for something you have been allowed to use. Even though a few artists might not require you to credit them, I think all of them will appreciate it if you do. But on a related note, crediting is -not- good enough for a copyrighted work. It's like telling your friend you are thanking him for the TV you stole. You still -stole- it. Saying you did doesn't make it better.
I'm not sure if this is at all helpful, but if I were you I'd either remove and replace content you don't have permission for, or ensure you do have permission. Personally, the first one is the safer and easier option. Your other option of releasing it anyway is not a very good idea, even if it seems easier than replacing and removing things. My word of advice is to make sure you have permission, or get permission before adding content you don't own. If you can't, don't add it and find something else workable instead. It's a much safer workflow than finishing something with the knowledge you might not be in the clear.