MPEG-FAQ 4.1:

What the problem with all these file extensions for MPEG-files ?

Hm, nobody standarized the file extensions yet, the are just common use.
the first MPEG-users (Xing) just used .mpg for a file extension, but
they had their special (non MPEG-standard) format. Then the "invented"
.mp2 for audio only files (well .mp2 looks more like MPEG-2, does it ?
Some had file-extension they wanted, some ignored MS-DOS file systems ...


The following extensions are there:
 
.mpg       could be everything ;o) usally only Xing-format (only I-frame)
.mps       MPEG-1 IPB Systemstream (video and audio)
.m1v       MPEG-1 IPB only video or systemstream
.mpv       MPEG-1 IPB only video (sometime even .vmp)
 
.mp2       MPEG-1 only audio (mostly layer 1 or 2)
.mpa       MPEG-1 only audio (mostly layer 1 or 2) (sometimes even .amp)
.l3        MPEG-1 only audio (layer 3)
 
.m2v       MPEG-2 IPB only video (is there some MPEG-2 audio out ?)

My own idea for file extension looks like this: 
 
.m1s       MPEG-1 IPB systemstream (video and audio)
.m1v       MPEG-1 IPB videostream
.m1a       MPEG-1 IPB audiostream

.m2s       MPEG-2 IPB systemstream (video and audio)
.m2v       MPEG-2 IPB videostream
.m2a       MPEG-2 IPB audiostream

There is no real need for a support of I-frame only streams anymore.
There are PD-players with IPB-frame support for every platform now
and the new players can play the old streams too.

Roman Czyborra is working on getting these extension registered for
WWW-use, so let's see ...

Look up the URL=http://www.cs.tu-berlin.de/~czyborra/registrations/ forcan
how this process is going on.