1. When two males make a baby together (without the help of a female). Usually used in some kind of fanfic.
2. Not a file type
girl: Squee, did you hear Branden and Sean had an mpreg baby together? They're so Kawaii!
boy: What, mpreg?! Ewww.
girl 1: Hey, did you get kawaiivideo.mpreg I sent you?
girl 2: Yeah! Hey wait, it doesn't open...
random dude: ...You realize that is not a file type, right? I think you mean kawaiivideo.mpeg.
girl 2: Gosh, you're right~!