Generate attachement to Post from featured image

I am using WP-JSON API Plugin.
If i choose an image as featured that is allready uploaded in my media library it does not generate attachements Object in JSON like the example bellow.

    attachments: [
{
id: 245613,
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02.jpg",
slug: "ek-france-02",
title: "ek-france-02",
description: "",
caption: "",
parent: 245612,
mime_type: "image/jpeg",
images: {
full: {
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02.jpg",
width: 770,
height: 400
},
thumbnail: {
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02-150x150.jpg",
width: 150,
height: 150
},
medium: {
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02-300x156.jpg",
width: 300,
height: 156
},
post-thumbnail: {
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02-615x400.jpg",
width: 615,
height: 400
},
slider-thumb: {
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02.jpg",
width: 770,
height: 400
},
large-thumb: {
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02-360x235.jpg",
width: 360,
height: 235
},
fb-thumb: {
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02-330x84.jpg",
width: 330,
height: 84
},
medium-thumb: {
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02-100x70.jpg",
width: 100,
height: 70
},
square-thumb: {
url: "mysite.com/wp-content/uploads/2015/06/ek-france-02-60x60.jpg",
width: 60,
height: 60
}
}
}
],

Is there a way to generate attachements from featured image per post from images not attached to my post?