Create a entity basefield as with a file type #drupal8

filetype.php

$fields['download_file'] = BaseFieldDefinition::create('file')
    ->setLabel(t('Forecast data'))
    ->setSetting('file_directory', 'forecast')
    ->setSetting('max_filesize', '10MB')
    ->setSetting('file_extensions', 'csv')
    ->setSetting('description_field', FALSE)
    ->setDisplayOptions('form', [
        'label' => 'hidden',
        'type' => 'file_generic',
        'weight' => 4,
    ])
    ->setDisplayConfigurable('form', TRUE)
    ->setDisplayConfigurable('view', TRUE);