The full form of mpg is short for MPEG (Motion Picture Experts Group). It’s used on Computing ,File Extensions in Worldwide

MPG is a file format that uses a format standardized by the Moving Pictures Experts Group (MPEG). In the old days, Windows 3.1 and DOS had an 8.3 naming convention: you could only have a maximum of 8 characters in the filename and a maximum of three characters in the extension. Hence the extension .mpg for mpeg. That wasn’t a limitation to, say, Unix users, who could name their files with .mpeg extensions if they wanted to. Even after Windows 95 and long filenames appeared, the .mpg extension stayed.