Read Bill Hunt on a file type as WRAPPER http://forums.adobe.com/thread/440037
What is a CODEC... a Primer http://forums.adobe.com/thread/546811
What CODEC is INSIDE that file? http://forums.adobe.com/thread/440037
Report back with the codec details of your file, use the programs below
For Mac http://mediainfo.massanti.com/
Please look at the links, that John T. posted. They will give you a lot of useful info on CODEC's, and probably most importantly, tell you how to find out which CODEC you are dealing with. That will be the key, and if you post that, someone here can tell you how to proceed.
My first guesses would be, and in this order:
There are answers to each of those.