Additional file entity fields in widget_form
You have to add it to the field type, not entity type.  Extend FileItem and override propertyDefinitions() to add it to the field properties. Like the description you've mentioned was added by FileItem extending EntityReferenceItem:

  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties = parent::propertyDefinitions($field_definition);

    $properties['display'] = DataDefinition::create('boolean')
      -&gt;setLabel(t('Display'))
      -&gt;setDescription(t('Flag to control whether this file should be displayed when viewing content'));

    $properties['description'] = DataDefinition::create('string')
      -&gt;setLabel(t('Description'));

    return $properties;
  }