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Contact steam support and ask them again? And perhaps you should write and say that you have read the Steam Forums ToS and their rules for Online Conduct. here and here
And that you've realized you where in breach of this and you will in the future adhere to these rules. But I think first you have to recognize that your ban was not wrongfully administrated.
You reposted leaked content. You did not upload it to the web, but you reposted it to steam. This was a clear breach of the rules of the Steam Forums. You were guilty as charged, accept that and ask them that you've learnt your lesson and wont be doing it in the future.
Where you are going about this wrong is saying "I dont think that is a problem" Well Steam does, and they make the rules, you have to adhere to their rules, not your own. Your chance to get unbanned lies with realizing you where rightfully banned but that you regret your actions, wont do it again adn then maybe they'll unban you.