If they released to other mirrors at the same time I don't think it would be an issue but if you are holding back to give a commercial file server first pickings, I think it's pretty clear.

Remember, fileplanet couldn't charge at all without content. Free content makes them money. If you can get it somewhere else then the appeal of their subscription service loses its luster. Even though it's only one of many files, they still need files to make money and the law will see each file as an individual case so why shouldn't we. A subscription service will no doubt work as hard as possible to get as many 'limited' exclusive files possible and may end up pressuring modmakers for exclusivity.

What is even scarier is who exactly is in breach? If the original game designers/publishers decide that an agreement like this is in breach of the EULA and you've made a verbal/written agreement with the file server for exclusivity then I think that the modmakers are going to end up with the short stick. If you made the agreement, it'll be your responsibility. Interestingly, this also applies to being published on the cover CDs in gaming magazines.

Basically, modmakers do all the work, hand over to people who'll try to make money and possibly still take all the blame for any breaches of copyright.

Cover yourselves, Never make any sort of exclusivity agreement! Even a Verbal one! They will keep copies of your E-mails! If you get on a payserver or magazine disc then make sure that the file is also available publically for free as well, even if just on a slow server. That way, you cannot be blamed for breaches of copyright in the distribution process and any problems would be between the publishers/designers & the offending distributor.

If you have already made some sort of agreement like this then see a lawyer. I'd recommend breaking it as no agreement between two parties can be deemed as legally binding if it breaks common law. If you find that you are in violation of the EULA then you would be legally required to uphold the stated copyright restrictions before you uphold personal agreements so I don't think that you'd get into any trouble by breaking the exclusivity agreement. Just as long as you notified the other party about the dissolution of the agreement immediately.

Still, I'm not a lawyer so see one if you are in doubt.