Add the file count field.
function tmgmt_update_8011() {
$storage_definition = BaseFieldDefinition::create('integer')
->setLabel(t('File count'))
->setSetting('unsigned', TRUE)
->setInitialValue(0);
\Drupal::entityDefinitionUpdateManager()
->installFieldStorageDefinition('file_count', 'tmgmt_job_item', 'tmgmt', $storage_definition);
}