BaseFieldDefinition to create file field

I create a custom module refered to "content_entity_example" module of to upload file.
is it possible to use BaseFieldDefinition here ?

This is my code

$validators = array(
'file_validate_extensions' => array('pdf doc docx xls xlsx ppt pptx png jpg jpeg'),
'file_validate_size' => array(file_upload_max_size()),
);
$fields['fid'] = BaseFieldDefinition::create('file')
  ->setLabel(t('File Name'))
  ->setDescription(t('The File of the associated event.'))
  //->setSetting('target_type', 'file')
  //->setSetting('handler', 'default')
  ->setSetting('upload_validators', $validators)
  ->setDisplayOptions('view', array(
	'label' => 'above',
	'type' => 'file',
	'weight' => -3,
  ))
  ->setDisplayOptions('form', array(
	'type' => 'file',
	'description' => array(										// doesn't work
		'theme' => 'file_upload_help', 							// doesn't work
		'description' => t('A Gettext Portable Object file.'),	// doesn't work
		),														// doesn't work
	'settings' => array(
		'upload_validators' => $validators,
	),
	'weight' => -3,
  ))
  ->setDisplayConfigurable('form', TRUE)
  ->setDisplayConfigurable('view', TRUE);

My problem is
1. using pdf doc docx xls xlsx ppt pptx png jpg jpeg file
- in description help informed Available extension is only txt
- i can upload file using extension above
- when submit button clicked i got error message that allowed extention is only txt.

2. using txt file
- i can not upload file and got an error that available extension are "pdf doc docx xls xlsx ppt pptx png jpg jpeg".

How to fix it ?
Thanks