I use the app OPlayerHD to view .avi files sent to me. You can get the Lite version, which is free at the app store, to seeif it works for you (it's $4.99 for the full-feature version).
It's just one of many, and I've got no stake in it - but I've never had any trouble opening an e-mailed file with this app. Of course, and as always, YMMV.
Marilyn