class FileFieldItemList

class FileFieldItemList extends EntityReferenceFieldItemList {
    
    /**
     * {@inheritdoc}
     */
    public function defaultValuesForm(array &$form, FormStateInterface $form_state) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function postSave($update) {
        $entity = $this->getEntity();
        if (!$update) {
            // Add a new usage for newly uploaded files.
            foreach ($this->referencedEntities() as $file) {
                \Drupal::service('file.usage')->add($file, 'file', $entity->getEntityTypeId(), $entity->id());
            }
        }
        else {
            // Get current target file entities and file IDs.
            $files = $this->referencedEntities();
            $ids = [];
            
            /** @var \Drupal\file\FileInterface $file */
            foreach ($files as $file) {
                $ids[] = $file->id();
            }
            // On new revisions, all files are considered to be a new usage and no
            // deletion of previous file usages are necessary.
            if (!empty($entity->original) && $entity->getRevisionId() != $entity->original
                ->getRevisionId()) {
                foreach ($files as $file) {
                    \Drupal::service('file.usage')->add($file, 'file', $entity->getEntityTypeId(), $entity->id());
                }
                return;
            }
            // Get the file IDs attached to the field before this update.
            $field_name = $this->getFieldDefinition()
                ->getName();
            $original_ids = [];
            $langcode = $this->getLangcode();
            $original = $entity->original;
            if ($original->hasTranslation($langcode)) {
                $original_items = $original->getTranslation($langcode)->{$field_name};
                foreach ($original_items as $item) {
                    $original_ids[] = $item->target_id;
                }
            }
            // Decrement file usage by 1 for files that were removed from the field.
            $removed_ids = array_filter(array_diff($original_ids, $ids));
            $removed_files = \Drupal::entityTypeManager()->getStorage('file')
                ->loadMultiple($removed_ids);
            foreach ($removed_files as $file) {
                \Drupal::service('file.usage')->delete($file, 'file', $entity->getEntityTypeId(), $entity->id());
            }
            // Add new usage entries for newly added files.
            foreach ($files as $file) {
                if (!in_array($file->id(), $original_ids)) {
                    \Drupal::service('file.usage')->add($file, 'file', $entity->getEntityTypeId(), $entity->id());
                }
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function delete() {
        parent::delete();
        $entity = $this->getEntity();
        // If a translation is deleted only decrement the file usage by one. If the
        // default translation is deleted remove all file usages within this entity.
        $count = $entity->isDefaultTranslation() ? 0 : 1;
        foreach ($this->referencedEntities() as $file) {
            \Drupal::service('file.usage')->delete($file, 'file', $entity->getEntityTypeId(), $entity->id(), $count);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteRevision() {
        parent::deleteRevision();
        $entity = $this->getEntity();
        // Decrement the file usage by 1.
        foreach ($this->referencedEntities() as $file) {
            \Drupal::service('file.usage')->delete($file, 'file', $entity->getEntityTypeId(), $entity->id());
        }
    }

}