Add hook to add a11y text to file media

web/themes/asc_bootstrap_bux/asc_bootstrap_bux.theme

@@ -1141,3 +1141,17 @@ function asc_bootstrap_bux_preprocess_views_exposed_form(&$variables)
     }
   }
 }
+
+/**
+ * Appends accessible text to File media when it opens in a new tab.
+ */
+function asc_bootstrap_bux_preprocess_file_link(array &$variables)
+{
+  $link = $variables['link'];
+  $attributes = $link->getUrl()->getOption('attributes');
+  if (isset($attributes['target']) && $attributes['target'] === '_blank') {
+    $string = $link->getText();
+    $string .= " (opens in a new window)";
+    $link->setText($string);
+  }
+}