File.php

<?php

namespace Drupal\ema_document\Plugin\media\Source;

use Drupal\file\FileInterface;
use Drupal\media\MediaInterface;
use Drupal\media\MediaTypeInterface;
use Drupal\media\MediaSourceBase;
use Drupal\media\Plugin\media\Source\File as FileBase;

/**
 * Extends the File entity media source.
 *
 * @see \Drupal\file\FileInterface
 *
 * @MediaSource(
 *   id = "file",
 *   label = @Translation("File"),
 *   description = @Translation("Use local files for reusable media."),
 *   allowed_field_types = {"file"},
 *   default_thumbnail_filename = "generic.png"
 * )
 */
class File extends FileBase {

  /**
   * Key for "Published" metadata attribute.
   *
   * @var string
   */
  const METADATA_ATTRIBUTE_DATE_PUBLISHED = 'published';

  /**
   * Key for "Updated" metadata attribute.
   *
   * @var string
   */
  const METADATA_ATTRIBUTE_DATE_UPDATED = 'update';

  /**
   * Key for "Example" metadata attribute.
   *
   * @var string
   */
  const METADATA_ATTRIBUTE_EXAMPLE_HERE = 'example';
  
  /**
   * {@inheritdoc}
   */
  public function getMetadataAttributes() {
    // Add attributes.
    return parent::getMetadataAttributes() + [
      static::METADATA_ATTRIBUTE_DATE_PUBLISHED => $this->t('Published'),
      static::METADATA_ATTRIBUTE_DATE_UPDATED => $this->t('Updated'),
      static::METADATA_ATTRIBUTE_EXAMPLE_HERE => $this->t('Example'),
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function getMetadata(MediaInterface $media, $attribute_name) {
    /** @var \Drupal\file\FileInterface $file */
    $file = $media->get($this->configuration['source_field'])->entity;
    // If the source field is not required, it may be empty.
    if (!$file) {
      return parent::getMetadata($media, $attribute_name);
    }
    switch ($attribute_name) {
      // Using the example attribute.
      case static::METADATA_ATTRIBUTE_EXAMPLE_HERE:
        return $file->getFilename();

      // Inherits the default behaviour
      default:
        return parent::getMetadata($media, $attribute_name);
    }
  }

  /**
   * {@inheritdoc}
   */
  public function createSourceField(MediaTypeInterface $type) {
    return parent::createSourceField($type)->set('settings', [
      'file_extensions' => 'pdf',
    ]);
  }

}

function ema_document_media_source_info_alter(array &$info) {
  if (isset($info['file'])) {
    $info['file']['class'] = '\Drupal\ema_document\Plugin\media\Source\File';
  }
}