File Extension MP21 File Format AXMEDIS MPEG -21 Object MP21 stands for AXMEDIS MPEG -21 Object. This was last updated in March 2012 By Margaret Rouse Browse File Extensions Alphabetically: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # Ask a Question. Find an Answer. Ask your peers a question about this file extension. Ask a Question What's the difference between MPEG and 4K? Win XP - How do I delete "zero byte" file where the second character of the name is a colon Who invented MPEG-2?