QuickTime - file not a movie file
I read a post below where someone was trying to open QT files they had created, and the error message read "This file is not a movie file." Well, I keep receiving the same message but I'm trying to open documents created by reliable sources. For example, this morning I am trying desperately to open the Quicken Update "Quicken 2007_R2Updater.dmg.sit". I don't know what type of files movie files are, but I don't know why this SHOULD be a movie file. However when I look at the available applications for opening the file with, Quicktime appears to the be the only viable option. Why does my QuickTime try to open files from the Internet, and what application can I be using instead?
Thanks!
P.S. I already read another post about being able to open movie files from the Internet in QuickTime, and followed the suggestions (installed RealPlayer, installed Perian, un and re-installed Adobe Flash...)
Mac OS X (10.4.11)