Migrating image with fields Working on a migrate task for images. On d7 and d8 I've file_entity with two fields, but values are not imported.

Here is my source json:

{
  "items": [
    {
      "id": "2417",
      "uuid": "b0ad2fca-d100-45bf-9423-2474f00af68b",
      "filename": "terremoto_irpinia.jpg",
      "filepath": "public://terremoto_irpinia.jpg",
      "url": "http://localhost/sites/default/files/terremoto_irpinia.jpg",
      "links": [
        {
          "url": "http://www.foggiacittaaperta.it/news/read/terremoto-irpinia-1980-il-ricordo-da-napoli-a-foggia-35-anni-dopo",
          "title": "http://www.foggiacittaaperta.it/news/read/terremoto-irpinia-1980-il-ricordo-da-n...",
          "attributes": [],
          "html": false,
          "query": null,
          "fragment": null,
          "absolute": true,
          "display_url": "http://www.foggiacittaaperta.it/news/read/terremoto-irpinia-1980-il-ricordo-da-n..."
        }
      ],
      "alt_text": "some text",
      "created": "1472139573",
      "stato": "Permanente",
      "created_year": "2016",
      "directory": "public://immagini/2016/terremoto_irpinia.jpg"
    },
]
}


On yml, this is my process:

source:
  plugin: url
  data_fetcher_plugin: http
  data_parser_plugin: json
  urls: http://localhost/modules/custom/migrando/files/download_immagini.json
  item_selector: items
  fields:
    -
      name: fid
      selector: id
    -
      name: filename
      selector: filename
    -
      name: uuid
      selector: uuid
    -
      name: filepath
      selector: directory
    -
      name: url
      selector: url
    -
      name: links
      selector: links
    -
      name: alt_text
      selector: alt_text

  ids:
    fid:
      type: string

process:
  uid:
    plugin: default_value
    default_value: 1
  filename: filename
  uuid: uuid
  status:
    plugin: default_value
    default_value: 1
  created: created
  alt: alt_text
  field_image_alt_text: alt_text
  field_fonte_immagine:
    plugin: iterator
    source: links
    process:
      uri: url
      title: title
      options: attributes
  uri:
    plugin: file_copy
    source:
      - url
      - filepath


Don't work field_image_alt_text and field_fonte_immagine fields.
What's wrong?

Thanks!
I don't see the destination configuration here - presumably the plugin is entity:file, correct? Are you setting default_bundle there, so the file entities are created with the correct type (the bundle those fields are attached to)?