Function sharing of JavaScript verification image type (extension)

function fucCheckJpgAndGif(strFileName) {
    if (strFileName != "") {
        var strtype = ( - 4, );
        strtype = ();
        if ((strtype == ".jpg") || (strtype == ".gif") || (strtype == "jpeg") || (strtype == ".bmp")) {
            return true;
        }
        else {
alert("Please upload pictures in jpg, gif or bmp format, thank you!");
            return false;
        }
    }
alert("Please upload images in jpg, gif or bmp format, thank you!");/* from /js/ */
    return false;

}