JavaScript function sharing to verify image type (extension)_javascript skills

OnClientClick="return fucCheckJpgAndGif( form1.File1.value);" --File1 is the upload file control

function fucCheckJpgAndGif(strFileName) {
if (strFileName != "") {
var strtype = strFileName.substring(strFileName.length - 4, strFileName.length);
strtype = strtype.toLowerCase();
if (( strtype == ".jpg") || (strtype == ".gif") || (strtype == "jpeg") || (strtype == ".bmp")) {
                  return true;
                                                                                                                                                                                                                                                                                                        ; please Upload pictures in jpg, gif or bmp format, thank you! ");/* from http://yige.org/js/ */
return false;

}