Attaching jpg files

A friend with a MAC has been trying to show me his latest digital picture. He claims it is a jpg file but when he attaches it to an e-mail and sends it, I get it as a "sit" file. Apparently this is a compressed file but my PC doesn't recognize it. I know nothing about MAC's but my friend is obviously doing something in his e-mail program to compress the file. Does anyone know how my friend can send a picture as a jpg so that I get it as a jpg?

Thanks,

Bob