How to migrate files together with additional fields In my Drupal 8.4 installation I use the module File Entity. In the module are defined a few additional fields: 'author', 'title', 'alt',... My migration works. The files are imported, but the additional fields are after the import empty.

The complete YML (modules\my_migration_module\config\install\migrate_plus.migration.files.yml) looks like:

langcode: en
status: true
id: files
migration_group: import_xml
label: 'Import the test XML file'
source:
  plugin: url
  data_fetcher_plugin: file
  urls: 'http://example.org/test.xml'
  data_parser_plugin: xml
  item_selector: /imports/files
  fields:
    -
      name: id
      label: ID
      selector: ./@id
    -
      name: title
      label: Title
      selector: ./@title
    -
      name: alt
      label: Alt
      selector: ./@alt
    -
      name: author
      label: Author
      selector: ./@author
    -
      name: filename
      label: Filename
      selector: ./@filename
    -
      name: uri
      label: URI
      selector: ./@uri
    -
      name: target
      label: Target
      selector: ./@target
  keys:
    - id
  ids:
    id:
      type: string
process:
  field_alt: alt
  field_title: title
  field_author: author
  filename: filename
  uri:
    plugin: download
    source:
      - uri
      - target
destination:
  plugin: 'entity:file'
  source_path_property: uri
  destination_path_property: target
template: null


E.g. for the field title I have tried:

field_title: title
title: title
field_title/field_title_value: title
field_title/field_title_value: title
field_title/field_title: title
file__field_titel/field_title_value: title
file__field_titel/field_title: title


I have found a solution for a similar problem in Drupal 7: How to migrate fields attached to files? I'm not sure, how can I implement this in Drupal 8.
The YML code is correct. But there is an issue in Drupal core: https://www.drupal.org/project/drupal/issues/2822545

There is also a patch. I have tested the patch and it works:
https://www.drupal.org/files/issues/2822545-cannot_update_file_properties-6.patch

The patch can be installed with Composer:

{
    ...
    "require": {
        "cweagans/composer-patches": "~1.0",
        "drupal/core": "8.4.6",
        ...
    },
    ...
    "extra": {
        "patches": {
           "drupal/core": {
             "Solve the issue when migrating files with additional fields": "https://www.drupal.org/files/issues/2822545-cannot_update_file_properties-6.patch"
            }
        }
    }
}