im trying to do file extension checking but its not working :(

i have the following in the head

Code:
<script language="JavaScript">
    extArray = new Array(".jpg", ".png", ".bmp");
    function LimitAttach(form, file) {
    allowSubmit = false;
    if (!file) return;
    while (file.indexOf("\\") != -1)
    file = file.slice(file.indexOf("\\")
...