How to migrate files together with additional fields
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"
            }
        }
    }
}