Drupal 8 File Field Target _Blank

If you prefer not to update core, use a hook_preprocess_file_link function in your theme file to add the target attribute on the generated link. Here's an example:

function hook_preprocess_file_link(&$variables) {
  if($variables['link'] && $variables['link'] instanceof \Drupal\Core\GeneratedLink){
    $doc = new DOMDocument();
    $doc->loadHTML($variables['link']->getGeneratedLink());
    $node = $doc->getElementsByTagName('a')[0];
    $node->setAttribute('target', '_blank');
    $variables['link']->setGeneratedLink($doc->saveHTML());
  }
}

By the time this hook runs, the file link HTML has already been generated. So, we have to parse the HTML into a DOMDocument object, set the target attribute and then write the modified HTML back out into the generated link.

I made a Field Formatter on a recent project for creating more configurable file download links. You get to choose whether or not to open the file in a new tab, whether the browser should add the download attribute to the link, and the link text. You could put this in a custom module at your_module/src/Plugin/Field/FieldFormatter. Depending on what link text you want to display, it might not work for you, but I thought it would be worth throwing out there.

<?php

namespace Drupal\your_module\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\file\Plugin\Field\FieldFormatter\FileFormatterBase;
use Drupal\Core\Url;
use Drupal\Core\Form\FormStateInterface;

/**
 * Plugin implementation of the 'file_download_link' formatter.
 *
 * @FieldFormatter(
 *   id = "file_download_link",
 *   label = @Translation("File Download Link"),
 *   weight = "100",
 *   field_types = {
 *     "file"
 *   }
 * )
 */
class FileDownloadLink extends FileFormatterBase {

  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    $options = parent::defaultSettings();
    $options['link_text'] = 'Download';
    $options['new_tab'] = TRUE;
    $options['force_download'] = TRUE;
    return $options;
  }

  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element = parent::settingsForm($form, $form_state);
    $element['link_text'] = [
      '#type' => 'textfield',
      '#title' => 'Link Text',
      '#default_value' => $this->getSetting('link_text'),
      '#description' => 'This text is linked to the file',
      '#required' => TRUE,
    ];
    $element['new_tab'] = [
      '#type' => 'checkbox',
      '#title' => 'Open file in new tab',
      '#default_value' => $this->getSetting('new_tab'),
    ];
    $element['force_download'] = [
      '#type' => 'checkbox',
      '#title' => 'Force Download',
      '#default_value' => $this->getSetting('force_download'),
      '#description' => 'This adds the <i>download</i> attribute to the link, which works in many modern browsers.',
    ];
    return $element;
  }

  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = ['Link text: ' . $this->getSetting('link_text')];
    if ($this->getSetting('new_tab')) {
      $summary[] = 'Open in new tab';
    }
    if ($this->getSetting('force_download')) {
      $summary[] = 'Force download';
    }
    return $summary;
  }

  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];

    foreach ($this->getEntitiesToView($items, $langcode) as $delta => $file) {

      //options for the link, like classes
      $file_type_explosion = explode("/", $file->getMimeType());
      $file_type = end($file_type_explosion);
      $options = [
        'attributes' => [
          'class' => [
            'file-download',
            'file-download-' . $file_type,
          ],
        ]
      ];
      if ($this->getSetting('new_tab')) {
        $options['attributes']['target'] = '_blank';
      }
      if ($this->getSetting('force_download')) {
        $options['attributes']['download'] = TRUE;
      }

      //make the render array
      $elements[$delta] = [
        '#type' => 'link',
        '#title' => $this->getSetting('link_text'),
        '#url' => Url::fromUri(file_create_url($file->getFileUri())),
        '#options' => $options,
        '#cache' => [
          'tags' => $file->getCacheTags(),
        ],
      ];
    }

    return $elements;
  }

}