MPEG - video
Is one of the basic file types File Juicer can extract from other files. MPEG can be embedded in QuickTime files and extracted without transcoding if this is the case.
MPG files were also once common on web pages for video, and they therefore ended up in browser cache files and web archives from which File Juicer could extract them. This use is now rare as most web sites have switched to mp4/h264 or Flash.
MPEG itself does not contain other file types.
MPEG files have a tiny header, and random data can look like an MPEG header. Therefore File Juicer will on occation detect MPEG files inside other files, even if it is not a real MPEG file. Verifying if an extracted file is a valid MPEG file is done by dropping it on VLC or QuickTime Player.
Mpeg-2 files may give an error (-2048) when you play it in QuickTime Player. Apple sell a $20 MPEG2 component for QuickTime.
Alternatively you can use VLC to view it.