Unknown file type

I have a file with avi extension, but GSpot reported that it is not an AVI file. How can I find out what type of file it is? It is a download file. It might be a fake media file. But how can I find out?
Thanks

i have downaoded a movie,m using windows xp,and i cant play the movie in vlc player,media player classic,dziobas rarplayer,the error occurs is unknown format,please help me with this