Just to be sure you understand, Temporay Internet Files is not a good place to save an email attachment if it's something you really want to keep. Temporay Internet Files get deleted if your clear your browser cache, run Windows Disk Cleanup, or run some other disk cleanup utility such as CCleaner. And, even if you don't manually activate a cleanup procedure, files may be automatically removed from Temporay Internet Files when they exceed a set amount of hard drive space.
Have you used Windows Search to search for the file?