The "attachment" type needs to be defined with all the attributes of attachments (captions, alt_text, etc).

We should compare the attributes that the REST API outputs, as well as what wp_prepare_attachment_for_json outputs

Based on my initial research, the data that core uses for images either as part of the main post object or in post_meta are:

• id
• post_name
• post_parent
• post_parent
• _edit_last
• _edit_lock
• _wp_attached_file
• _wp_attachment_backup_sizes
• _wp_attachment_context
• _wp_attachment_image_alt
• _wp_attachment_metadata
• _wp_old_slug
• comment_count
• comment_status
• guid
• menu_order
• mime_type
• ping_status
• pinged
• post_author
• post_content
• post_content_filtered
• post_date
• post_date_gmt
• post_excerpt
• post_modified
• post_modified_gmt
• post_password
• post_status
• post_title
• post_type
• to_ping

The wp_prepare_attachment_for_json function returns the following fields:

• id
• title
• filename
• url
• link
• alt
• author
• description
• caption
• name
• status
• uploadedTo
• date
• modified
• menuOrder
• mime
• type
• subtype
• icon
• dateFormatted
• nonces
• editLink
• sizes
• width
• height
• fileLength
• compat