file upload control validation in javascript

FileUpload1.Attributes.Add("onchange", "return checkFileExtension(this);")




 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();
   
        validExtensions[0] = 'jpg';
        validExtensions[1] = 'jpeg';
        validExtensions[2] = 'bmp';
        validExtensions[3] = 'png';
        validExtensions[4] = 'gif';
        validExtensions[5] = 'tif';
        validExtensions[6] = 'tiff';
        validExtensions[7] = 'txt';
        validExtensions[8] = 'doc';
        validExtensions[9] = 'xls';
        validExtensions[10] = 'pdf';
   
        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;
    }