HAL normalization of file fields don't provide file entity id or file entity REST URL

Problem/Motivation

When using the hal_json format with an entity that has a file field the hal_json output on GET does not provide enough information to access the file entity via the REST API.

I discovered this while reviewing #1927648: Allow creation of file entities from binary data via REST requests

I wanted to GET a node with a file field then be able to either GET, POST, PATCH or DELETE the file referenced. The example was working with had both a image and file field.

Here is the node GET response, the example has an image field, a public:// file field and a private:// file field.

{
  "_links": {
    "self": {
      "href": "http://d8.dev/node/37?_format=hal_json"
    },
    "type": {
      "href": "http://d8.dev/rest/type/node/article"
    },
    "http://d8.dev/rest/relation/node/article/revision_uid": [
      {
        "href": "http://d8.dev/user/1?_format=hal_json"
      }
    ],
    "http://d8.dev/rest/relation/node/article/uid": [
      {
        "href": "http://d8.dev/user/1?_format=hal_json",
        "lang": "en"
      }
    ],
    "http://d8.dev/rest/relation/node/article/field_image": [
      {
        "href": "http://d8.dev/sites/default/files/2017-09/webform1.png",
        "lang": "en"
      }
    ],
    "http://d8.dev/rest/relation/node/article/field_private_file": [
      {
        "href": "http://d8.dev/system/files/2017-09/interdiff-109-115.txt"
      }
    ],
    "http://d8.dev/rest/relation/node/article/field_public_file": [
      {
        "href": "http://d8.dev/sites/default/files/2017-09/interdiff-275-277.txt"
      }
    ],
    "http://d8.dev/rest/relation/node/article/field_tags": [
      {
        "href": "http://d8.dev/taxonomy/term/1?_format=hal_json",
        "lang": "en"
      }
    ]
  },
  "nid": [
    {
      "value": 37
    }
  ],
  "uuid": [
    {
      "value": "aef1528d-aedb-4afc-a20a-755a71805937"
    }
  ],
  "vid": [
    {
      "value": 53
    }
  ],
  "langcode": [
    {
      "value": "en",
      "lang": "en"
    }
  ],
  "type": [
    {
      "target_id": "article"
    }
  ],
  "revision_timestamp": [
    {
      "value": "2017-09-07T19:33:55+00:00",
      "format": "Y-m-d\\TH:i:sP"
    }
  ],
  "_embedded": {
    "http://d8.dev/rest/relation/node/article/revision_uid": [
      {
        "_links": {
          "self": {
            "href": "http://d8.dev/user/1?_format=hal_json"
          },
          "type": {
            "href": "http://d8.dev/rest/type/user/user"
          }
        },
        "uuid": [
          {
            "value": "3d76e74e-25b5-45e9-af89-3d94f253b46c"
          }
        ]
      }
    ],
    "http://d8.dev/rest/relation/node/article/uid": [
      {
        "_links": {
          "self": {
            "href": "http://d8.dev/user/1?_format=hal_json"
          },
          "type": {
            "href": "http://d8.dev/rest/type/user/user"
          }
        },
        "uuid": [
          {
            "value": "3d76e74e-25b5-45e9-af89-3d94f253b46c"
          }
        ],
        "lang": "en"
      }
    ],
    "http://d8.dev/rest/relation/node/article/field_image": [
      {
        "_links": {
          "self": {
            "href": "http://d8.dev/sites/default/files/2017-09/webform1.png"
          },
          "type": {
            "href": "http://d8.dev/rest/type/file/file"
          }
        },
        "uuid": [
          {
            "value": "ce48df6b-83e9-47b8-9875-c2fe391ef9b1"
          }
        ],
        "uri": [
          {
            "value": "http://d8.dev/sites/default/files/2017-09/webform1.png"
          }
        ],
        "lang": "en"
      }
    ],
    "http://d8.dev/rest/relation/node/article/field_private_file": [
      {
        "_links": {
          "self": {
            "href": "http://d8.dev/system/files/2017-09/interdiff-109-115.txt"
          },
          "type": {
            "href": "http://d8.dev/rest/type/file/file"
          }
        },
        "uuid": [
          {
            "value": "50ab6ba6-7a86-4027-a86d-a26ebf0e09b5"
          }
        ],
        "uri": [
          {
            "value": "http://d8.dev/system/files/2017-09/interdiff-109-115.txt"
          }
        ]
      }
    ],
    "http://d8.dev/rest/relation/node/article/field_public_file": [
      {
        "_links": {
          "self": {
            "href": "http://d8.dev/sites/default/files/2017-09/interdiff-275-277.txt"
          },
          "type": {
            "href": "http://d8.dev/rest/type/file/file"
          }
        },
        "uuid": [
          {
            "value": "bb9ed1d1-f613-4647-b932-2e2f52143bfd"
          }
        ],
        "uri": [
          {
            "value": "http://d8.dev/sites/default/files/2017-09/interdiff-275-277.txt"
          }
        ]
      }
    ],
    "http://d8.dev/rest/relation/node/article/field_tags": [
      {
        "_links": {
          "self": {
            "href": "http://d8.dev/taxonomy/term/1?_format=hal_json"
          },
          "type": {
            "href": "http://d8.dev/rest/type/taxonomy_term/tags"
          }
        },
        "uuid": [
          {
            "value": "5228c13c-265a-42d1-bbca-ea4a30385cfc"
          }
        ],
        "lang": "en"
      }
    ]
  },
  "status": [
    {
      "value": true,
      "lang": "en"
    }
  ],
  "title": [
    {
      "value": "uuuuu",
      "lang": "en"
    }
  ],
  "created": [
    {
      "value": "2017-09-06T14:22:27+00:00",
      "lang": "en",
      "format": "Y-m-d\\TH:i:sP"
    }
  ],
  "changed": [
    {
      "value": "2017-09-07T19:33:55+00:00",
      "lang": "en",
      "format": "Y-m-d\\TH:i:sP"
    }
  ],
  "promote": [
    {
      "value": true,
      "lang": "en"
    }
  ],
  "sticky": [
    {
      "value": false,
      "lang": "en"
    }
  ],
  "default_langcode": [
    {
      "value": true,
      "lang": "en"
    }
  ],
  "revision_translation_affected": [
    {
      "value": true,
      "lang": "en"
    }
  ],
  "path": [
    {
      "alias": null,
      "pid": null,
      "langcode": "en",
      "lang": "en"
    }
  ],
  "content_translation_source": [
    {
      "value": "und",
      "lang": "en"
    }
  ],
  "content_translation_outdated": [
    {
      "value": false,
      "lang": "en"
    }
  ],
  "comment": [
    {
      "status": 2,
      "cid": 0,
      "last_comment_timestamp": 1504707773,
      "last_comment_name": null,
      "last_comment_uid": 1,
      "comment_count": 0,
      "lang": "en"
    }
  ]
}

Nowhere in the response do I have access to the file entity's id or url to access the file entity via the REST API.
This not true for the other entity reference fields.
For the term I am given: "href": "http://d8.dev/taxonomy/term/1?_format=hal_json
and for the user I am given: "href": "http://d8.dev/taxonomy/term/1?_format=hal_json

Therefore I wanted to view, update or delete the file entity I would not be able to via REST.

Proposed resolution

Not sure but somehow return http://d8.dev/entity/file/1?_format=hal_json

Remaining tasks

User interface changes

API changes

Response for hal_json normalized file fields would be changed.

Data model changes