--- /dev/null
+<?php
+
+namespace Drupal\Tests\file\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests multiple file upload.
+ *
+ * @group file
+ */
+class MultipleFileUploadTest extends BrowserTestBase {
+
+ /**
+ * {@inheritdoc}
+ */
+ public static $modules = ['file'];
+
+ /**
+ * {@inheritdoc}
+ */
+ protected function setUp() {
+ parent::setUp();
+
+ $admin = $this->drupalCreateUser(['administer themes']);
+ $this->drupalLogin($admin);
+ }
+
+ /**
+ * Tests multiple file field with all file extensions.
+ */
+ public function testMultipleFileFieldWithAllFileExtensions() {
+ $theme = 'test_theme_settings';
+ \Drupal::service('theme_handler')->install([$theme]);
+ $this->drupalGet("admin/appearance/settings/$theme");
+
+ $edit = [];
+ // Create few files with non-typical extensions.
+ foreach (['file1.wtf', 'file2.wtf'] as $i => $file) {
+ $file_path = $this->root . "/sites/default/files/simpletest/$file";
+ file_put_contents($file_path, 'File with non-default extension.', FILE_APPEND | LOCK_EX);
+ $edit["files[multi_file][$i]"] = $file_path;
+ }
+
+ // @todo: Replace after https://www.drupal.org/project/drupal/issues/2917885
+ $this->drupalGet("admin/appearance/settings/$theme");
+ $submit_xpath = $this->assertSession()->buttonExists('Save configuration')->getXpath();
+ $client = $this->getSession()->getDriver()->getClient();
+ $form = $client->getCrawler()->filterXPath($submit_xpath)->form();
+ $client->request($form->getMethod(), $form->getUri(), $form->getPhpValues(), $edit);
+
+ $page = $this->getSession()->getPage();
+ $this->assertNotContains('Only files with the following extensions are allowed', $page->getContent());
+ $this->assertContains('The configuration options have been saved.', $page->getContent());
+ $this->assertContains('file1.wtf', $page->getContent());
+ $this->assertContains('file2.wtf', $page->getContent());
+ }
+
+}