image type validation

i have a upload form and i would like to validate to see if the file is an image or not. Is there something I can add to this that would do the trick? I wonly want jpegs.

function validateSignup(frm) {

if (frm.UserName.value == '') {
alert("You must enter your UserName.");
frm.UserName.focus();
return false;
}

}

http://codewalkers.com/tutorials/35/1.html

thanks for the reply but i can not see how that would help me as i would like to do this client side via javascript. I have this witch will tell me if an image has been selected or not but would also like just jpegs.

function validateSignup(frm) {

if (frm.Image.value == '') {
alert("You must select a Image.");
frm.Image.focus();
return false;
}

Anyone know of if this can be done?

}