我正在尝试在metabox中使用media manager，我需要使用multiple select。我只是对从那个选择中输出有问题。我需要metabox中的元素来保存所有选定的文件URL。我的代码在这里：

jQuery(document).ready(function($){
var custom_uploader;
$(\'#upload_image_button\').click(function(e) {
    e.preventDefault();
    //If the uploader object has already been created, reopen the dialog
    if (custom_uploader) {
        custom_uploader.open();
        return;
    }
    //Extend the wp.media object
    custom_uploader = wp.media.frames.file_frame = wp.media({
        title: \'Choose Image\',
        button: {
            text: \'Choose Image\'
        },
        multiple: true
    });
    custom_uploader.on(\'select\', function() {
        selection.map( function( attachment ) {
        attachment = attachment.toJSON();
        $("#obal").after("<img src=" +attachment.url+">");
        });
    });
    custom_uploader.open();
});
});