Featured image doesn't attach to post

[bi0xid]

When you use an image as featured, it doesn't attach to the post.

Possibly related with #14390 (post image (thumbnail) in [gallery])

Tests:

• If the image is already attached, it doesn't reattach ✔

• If the image is uploaded while writing the post, it attaches like another image, even if you are not using it in the post. ✔

• If an image is in the media library, not attached, and you use it in your post, it automatically attaches to the post. ✔

• If the image is in the media library, not attached, and you use it as featured image in a post, it doesn't attach ✘