Add description in file_import

Already available

field_files:
plugin: file_import
source: uri
destination: file_destination
uid: uid
skip_on_missing_source: true
field_files/description: description

@michiellucas, can you please format the code? Or add some documentation?

field_files:
  plugin: file_import
  source: uri
  destination: file_destination
  uid: uid
  skip_on_missing_source: true
field_files/description: description

Is this the right syntax?

After a while I made my own solution since #5 won't work with a multi-valued field.

In my import process section I managed to get two arrays one for remote URLs and one for file descriptions:

_attachments_urls_array = [
  0 => 'https://oldsite.com/path/to/files/attachment_1.pdf',
  1 => 'https://oldsite.com/path/to/files/attachment_2.zip',
  2 => 'https://oldsite.com/path/to/files/attachment_3.mp3',
];

_attachments_labels_array = [
  0: 'My PDF document',
  1: 'My ZIP archive',
  2: 'My song',
]

First one is the source of my field_attachments multivalue file field working nice using file_import plugin from Migrate files (extended) module.

To apply file descriptions I figured how to patch the module but changes were too many and I walked to another solution.

What I did is an implementation of the PRE_ROW_SAVE event (ref. MigrateEvents).

Of course you need a custom module but you can use the one where you have your migration configurations.

First add a custom event subscriber (src/EventSubscriber/YourModuleSubscriber.php):

namespace Drupal\your_module\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Drupal\migrate\Event\MigrateEvents;
use Drupal\migrate\Event\MigrateImportEvent;
use Drupal\migrate\Event\MigratePreRowSaveEvent;

/**
 * Your Migrations event subscriber.
 */
class YourModuleSubscriber implements EventSubscriberInterface {

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    return [
      MigrateEvents::PRE_ROW_SAVE => ['onMigratePreRowSave'],
    ];
  }

  /**
   * Custom migration comtuting.
   *
   * @param \Drupal\migrate\Event\MigratePreRowSaveEvent $event
   *   The import event object.
   */
  public function onMigratePreRowSave(MigratePreRowSaveEvent $event) {
    $migration_id = $event->getMigration()->getBaseId();

    switch ($migration_id) {
      case 'your_migration_id':
        $row = $event->getRow();
        $destination = $row->getDestination();
        if (isset($destination['field_attachments'])) {
          $field_attachments = $destination['field_attachments'];
          foreach ($field_attachments as $index => $attachment) {
            if (isset($destination['_attachments_labels_array'][$index])) {
              $field_attachments[$index]['description'] = $destination['_attachments_labels_array'][$index];
            }
          }
          $row->setDestinationProperty('field_attachments', $field_attachments);
        }
        break;
      default:
        break;
    }
  }

}

Then add the related service in your module services file (create one if you don't have one) your_module.services.yml:

services:
  your_module.event_subscriber:
    class: Drupal\your_module\EventSubscriber\YourModuleSubscriber
    tags:
      - { name: event_subscriber }

I hope this helps.