Why https://www.drupal.org/node/2640842 is not a good idea

CustomFile.php

<?php // {custom_module}/src/Plugin/migrate/process/CustomFile.php Replace {custom_nodule} with your module's machine name

namespace Drupal\{custom_module}\Plugin\migrate\process; // Replace {custom_nodule} with your module's machine name

use Drupal\migrate\Plugin\MigrationInterface;
use Drupal\file\Plugin\migrate\process\d6\CckFile;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * @MigrateProcessPlugin(
 *   id = "custom_file"
 * )
 */
class CustomFile extends CckFile {

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
    $migration_plugin_configuration = [
      'source' => ['fid'],
      'migration' => 'custom_files', // Tthis is the only overriden line from CckFile
    ];

    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $migration,
      $container->get('plugin.manager.migrate.process')->createInstance('migration', $migration_plugin_configuration, $migration)
    );
  }

}

migrate_plus.migration.custom_files.yml

# {custom_module}/config/install/migrate_plus.migration.custom_files.yml
# customized /core/modules/file/migration_templates/d6_file.yml

id: custom_files
label: Custom files
source:
  plugin: d6_file
destination:
  plugin: 'entity:file'
  urlencode: true
  source_path_property: filepath
process:
  uid: # unlike in d6_file.yml, we want uids to default to 1. This is the main reason I created a custom file migration.
    plugin: default_value
    default_value: 1
  filename: filename
# fid: we omit it, let fid autoincrement; we don't need to keep fids from d6. Therefore we can't use the same fids as in D6.
# Because of this, we need to override the CckFile object and specify that d6 fids be mapped to the autoincremented fids
# created by this migration.
  uri:
    plugin: file_uri
    source:
      - filepath
      - file_directory_path
      - temp_directory_path
      - is_public
  filemime: filemime
  filesize: filesize
  status: status
  changed: timestamp
template: null
migration_dependencies:
  optional:
    - d6_file

migrate_plus.migration.custom_node.yml

# {custom_module}/config/install/migrate_plus.migration.custom_node.yml

id: custom_node
label: Custom node

migration_dependencies:
  required:
    - custom_files

source:
  plugin: d6_node

destination:
  plugin: entity:node

process:
  nid: nid
  vid: vid
  type: type
  langcode:
    plugin: default_value
    default_value: en
  title: title
  created: created
  changed: changed
  promote: promote
  sticky: sticky

  field_example_file:
    plugin: custom_file # Exactly same as d6_file, with the exeption that migration id mapping is based on the custom_file migration.
    source: field_d6_source_file