XnView Software

I know this has worked with previous versions of XnView but for some reason XnView 1.93.6 isn't creating thumbnails for MP4 files or playing them either!

I have the latest FFDShow (mp4 and aac enabled) and Nero 7 installed.
In Custom File Types, 3GP DIVX MP4 are all enabled.

Details of the MP4 file I'm trying to play...

Video: MPEG4 Video (H264) 384x288 25.00fps [GPAC ISO Video Handler]
Audio: AAC 16000Hz mono 30Kbps [GPAC ISO Audio Handler]

LMAO, "Photo Commander 5" plays the video fine but can't handle FLV videos where XnView can play them

Nevermind, I fixed it by renaming the MP4 to MKV which I think is legal given the codecs involved.

Chnaging a file extension is a bandaid fix at best (unless the file was misnamed in the first place. You can use MediaInfo to find out for certain what codecs are used & what container: http://mediainfo.sourceforge.net/ ).
As far as what containers/extensions are valid for h264/aac, it's pretty much all of them. Mov would be a more accurate choice since the mp4 container is based on the mov container, but pretty much anything other than mpg/mpeg would work. (Actually, mpg/mpeg would work for the aac portion, since aac was introduced in the mpeg-2 format, but I don't think the h264 would work in an mpeg.) The mkv format is a universal container, it can hold pretty much anything (with the possible exceptions of proprietary stuff like real & windows media). Avi can also hold nearly anything.