Make attachment mandatory based on a variable from Catalog Item

Hi everyone! I'm fairly new to ServiceNow. I'm having difficulties in making the attachment mandatory based from the value of a variable in Catalog Item.

What I wanted to achieve is when I select Create a New Service, attachment becomes mandatory. 

On the same catalog item, I was able auto populate the fields based on the Service Name selected using script include and GlideAjax. What I wanted to achieve is to make mandatory attachment field on a catalog item to be true when Create a New Service is selected and false when not selected using script include and GlideAjax.

Here's my code using script include and GlideAjax.

Script Include

var getService6 = Class.create();
getService6.prototype = Object.extendsObject(AbstractAjaxProcessor, {

	getMandatoryDetails: function() {
        var service6 = this.getParameter('sysparm_user');
        var gr = new GlideRecord('sc_cat_item');
        gr.addQuery('sys_id', service6);
        gr.query();
        // Query user records
        if (gr.next()) {
            gr.mandatory_attachment = true;
            gr.update();
        }
    },
    type: 'getService6'
});

Catalog Client Script

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading || newValue == '') {
        return;
    }

    var ga = new GlideAjax('getService6'); // Script Include Name
    ga.addParam('sysparm_name', 'getMandatoryDetails'); // Specific Function Name within the Script Include
    ga.addParam('sysparm_user', g_form.getValue('select_type_of_request')); // Value of the the field
    ga.getXML(setUpdate);

    function setUpdate(response) {
        var answer = response.responseXML.documentElement.getAttribute("answer");
        g_form.setValue('mandatory_attachment', answer);
    }

}

Any insights will be very helpful in understanding script include and GlideAjax. Thank you!