When I select jpeg in the Save As dialogue box the extension .iff appears instead. Help.
Sounds like you made the very bad mistake of copying one or more plug-ins from a prior version or of pointing Photoshop to a prior version's plug-in folder.
That is a HUGE no-no.
Now go delete any such copied plug-ins and uncheck the addtional plug-in folder.
Station_two has the right idea.
But just to be completely accurate, it can sometimes be okay to copy 3rd party plug-ins if there's no other way to get a CS6-compatible version.
What's an absolute no-no is to copy or link-to any of the plug-ins that Adobe has supplied with a different version of Photoshop. That conflict is what's causing this problem.
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