@Kevin: Looking into this a bit more it appears that Mail's sandbox in 10.8 prevents apps from opening files that have been downloaded into the symlinked Downloads folder. I'm not sure what – if any – solution there is to this at the moment other than just opening files only from the Downloads folder rather than mail! I'll look into it a bit further though, thanks for the heads up! I had seen this happen but had thought it was just my machine behaving weird!