Validate File Extensions using Javascript

function checkFileExtension(elem)
            {
            var filePath = elem.value;

            if(filePath.indexOf('.') == -1)
                return false;
           
            var validExtensions = new Array();
            var ext = filePath.substring(filePath.lastIndexOf('.') + 1).toLowerCase();
            //Add valid extentions in this array
            validExtensions[0] = 'xls';
            validExtensions[1] = 'xlsx';
            for(var i = 0; i < validExtensions.length; i++) {
                if(ext == validExtensions[i])
                    return true;
            }
            
            alert('The file extension ' + ext.toUpperCase() + ' is not allowed!');
            return false;
        }

FileUpload1.Attributes.Add("onchange", "checkFileExtension(" + FileUpload1.ClientID + ")");