JSON API not providing image alt-tag (Accessibility Issue)

#### Steps to reproduce the issue
Calling JSON REST API https://public-api.wordpress.com/rest/v1.1/sites/$site/posts/
It will produce a list of recent posts as well as links to its attached images. On the sections that produce information related to attachments, it does not produce Alt Tag, Title or Captions. When relying on API feed to embed contents from Wordpress into another website, it will generate an Accessibility issue as alt tag for images could not be rendered.

#### What I expected

I expect API to produce detailed metadata on the attachment, especially alt tag.

#### What happened instead

API only produce size and MIME information on the image attachment. It did not produce alt tag or title information.

Start by reproducing the response from the WordPress.com JSON REST API posts endpoint and inspect how attached images are represented. The change is complete when attachment data includes alt text, title, and captions so consuming sites can render accessible images.

Written by the indexing model from the issue text.