user2876: I'm uploading a file to an external system that accepts only mpg or mpeg extension files. I had a mp4 file so I changed the mp4 extension to mpg and I was able to upload the file into the system. Would other using the system who have access to this uploaded mpg file be able to view my video correctly? 
Why is it that I can change the extension from .mp4 to .mpg and the video still plays fine? Should it not give me an error because the extension is not correct?

user1898: The .mpg and .mpeg extensions are typically associated with MPEG-1 and MPEG-2 files. The structure of these files is different than the .mp4 format used for H.264 video, part of the MPEG-4 family of formats.
I suspect VLC and WMP can play the file because they must not be using the file extension to determine file type, they probably parse the file with all the supported parsers until one works. To prove this point, I took a .wmv movie and renamed it to .mp4. When I played it in my Windows 7 PC with WMP I got a warning window saying that the extension did not match the format, but I told it to proceed anyway and the file played just fine.
If this online service requires MPEG-1 or 2 then you should transcode your video to one of those formats before uploading. Unless the online service transcodes all uploaded videos to some standard format. In that case it does not really matter what the format of the uploaded video is, as long as the service can read it and transcode to its own favorite format.
Good luck.

2876: <p>I'm uploading a file to an external system that accepts only mpg or mpeg extension files. I had a mp4 file so I changed the mp4 extension to mpg and I was able to upload the file into the system. Would other using the system who have access to this uploaded mpg file be able to view my video correctly? </p>

<p>Why is it that I can change the extension from .mp4 to .mpg and the video still plays fine? Should it not give me an error because the extension is not correct?</p>
 1898: <p>The <code>.mpg</code> and <code>.mpeg</code> extensions are typically associated with MPEG-1 and MPEG-2 files. The structure of these files is different than the .mp4 format used for H.264 video, part of the MPEG-4 family of formats.</p>

<p>I suspect VLC and WMP can play the file because they must not be using the file extension to determine file type, they probably parse the file with all the supported parsers until one works. To prove this point, I took a .wmv movie and renamed it to .mp4. When I played it in my Windows 7 PC with WMP I got a warning window saying that the extension did not match the format, but I told it to proceed anyway and the file played just fine.</p>

<p>If this online service requires MPEG-1 or 2 then you should transcode your video to one of those formats before uploading. Unless the online service transcodes all uploaded videos to some standard format. In that case it does not really matter what the format of the uploaded video is, as long as the service can read it and transcode to its own favorite format.</p>

<p>Good luck.</p>
