Use IMCE files browser for custom field on Drupal 8/9

You can use IMCE to select a file.

if (\Drupal::moduleHandler()->moduleExists('imce') && \Drupal\imce\Imce::access()) {

  $form['values']['image-imce-field'] = [

    '#type' => 'textfield',

    '#title' => $this->t('Image'),

    '#attached' => ['library' => ['imce/drupal.imce.input']],

    '#attributes' => ['class' => ['imce-url-input']],

    '#default_value' => '',

  ];

}

Convert file path to FID

public static function getFidFromPath($path) {

  $uri = str_replace('/sites/default/files/', 'public://', $path);

  $query = \Drupal::entityQuery('file');

  $query->condition('uri', $uri);

  $query->sort('created', 'DESC');

  $fids = $query->execute();

  if ($fids) {

    $fids = array_values($fids);

    return $fids[0] ??  NULL;

  }

  return NULL;

}