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bug 27021) Session key was changed somewhere from an int to a string, which broke stashed uploads totally. Changed $wgRequest->getInt to $wgRequest->getText. The API never explicitly defined it as int, so should not be affected.
Note that this also may break API clients in certain cases for strongly, statically typed languages (e.g. int sessionKey = request->sessionKey;). I see no way around this, so I will announce a breaking change on the API mailing list shortly.