candyangel try unchecking "Enable QuickTime Exchange" under the QuickTime Exchange section in the QuickTime Settings control panel. When I was testing this I was told I would need to restart for this change to take. I didn't bother with that so this may not do what you want but if there is an option for what you're asking for this sounds like it.

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And then my computer crashed. Looks like it works. When you add .mp3 or whatever to the end of a file now it doesn't change it to QuickTime.