Drupal 6 "d6_field_file" migration plugin static lookup, leads creating empty file fields

Problem/Motivation

In line 76 of the "FieldFile" class in "web/core/modules/file/src/Plugin/migrate/process/d6/FieldFile.php" the $lookup_result will always result in an empty array, if the migrations are defined using migrate upgrade + migrate tools with the "--configure-only" flag.

This is because $this->migrateLookup->lookup() does NOT use "An array of migration plugin IDs to look up, or a single ID as a string"(source: method documentation) as the parameter but instead the migration ID, which should be correctly "upgrade_d6_file" instead of "d6_file" (current value) when previously generated using drush migrate-upgrade --legacy-db-key=migrate --configure-only.

This leads to all file fields not having any files attached, when migrated.

Steps to reproduce

• Generate migrations via drush migrate-upgrade --legacy-db-key=migrate --configure-only.
• Make sure your source db has entity bundles using file fields
• Run drush migrate-import --group=my_migration_group --continue-on-failure

Proposed resolution

Dynamically use the correct ID of the generated migration file.

Workaround:
See MR:
Change this line:

$lookup_result = $this->migrateLookup->lookup('d6_file', [$value['fid']]);

to

$lookup_result = $this->migrateLookup->lookup('upgrade_d6_file', [$value['fid']]);

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet