Quicktime and AVI Files
Quicktime says it supports AVI videos, but when I try to play mine it says I need to download a codec?
Mac Pro, iOS 6.1
There are multiple types of AVI encoders, and as such not all AVI files are recognised by QT.
The easiest thing to do is to move away from QT, and install VLC, which handles nearly all types of files.
http://www.videolan.org/vlc/download-macosx.html
Good luck.