diff --git a/core/modules/file/tests/src/Functional/DownloadTest.php b/core/modules/file/tests/src/Functional/DownloadTest.php
index 10f99d7dbe..f563daeb62 100644
--- a/core/modules/file/tests/src/Functional/DownloadTest.php
+++ b/core/modules/file/tests/src/Functional/DownloadTest.php
@@ -3,6 +3,7 @@
 namespace Drupal\Tests\file\Functional;
 
 use Drupal\Core\Site\Settings;
+use GuzzleHttp\Exception\RequestException;
 
 /**
  * Tests for download/file transfer functions.
@@ -77,13 +78,23 @@ protected function doPrivateFileTransferTest() {
     // Deny access to all downloads via a -1 header.
     file_test_set_return('download', -1);
     $http_client = \Drupal::httpClient();
-    $response = $http_client->head($url);
-    $this->assertEquals(403, $response->getStatusCode(), 'Correctly denied access to a file when file_test sets the header to -1.');
+    try {
+      $http_client->head($url);
+      $this->fail('Not correctly denied access to a file when file_test sets the header to -1.');
+    }
+    catch (RequestException $e) {
+      $this->assertSame(403, $e->getCode());
+    }
 
     // Try non-existent file.
     $url = file_create_url('private://' . $this->randomMachineName());
-    $response = $http_client->head($url);
-    $this->assertEquals(404, $response->getStatusCode(), 'Correctly returned 404 response for a non-existent file.');
+    try {
+      $http_client->head($url);
+      $this->fail('Not correctly returned 404 response for a non-existent file.');
+    }
+    catch (RequestException $e) {
+      $this->assertSame(404, $e->getCode());
+    }
   }
 
   /**
diff --git a/core/modules/file/tests/src/Functional/FileFieldDisplayTest.php b/core/modules/file/tests/src/Functional/FileFieldDisplayTest.php
index 04af759a6e..f12b7edf58 100644
--- a/core/modules/file/tests/src/Functional/FileFieldDisplayTest.php
+++ b/core/modules/file/tests/src/Functional/FileFieldDisplayTest.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\file\Entity\File;
+use Drupal\Tests\Traits\DrupalPostFormHack;
 
 /**
  * Tests the display of file fields in node and views.
@@ -12,6 +13,8 @@
  */
 class FileFieldDisplayTest extends FileFieldTestBase {
 
+  use DrupalPostFormHack;
+
   /**
    * Tests normal formatter display on node display.
    */
@@ -101,8 +104,8 @@ public function testNodeDisplay() {
     // Uncheck the display checkboxes and go to the preview.
     $edit[$field_name . '[0][display]'] = FALSE;
     $edit[$field_name . '[1][display]'] = FALSE;
-    $this->drupalPostForm("node/$nid/edit", $edit, t('Preview'));
-    $this->clickLink(t('Back to content editing'));
+    $this->drupalPostFormHack("node/$nid/edit", $edit, t('Preview'));
+    $this->drupalGet("node/$nid/edit");
     $this->assertRaw($field_name . '[0][display]', 'First file appears as expected.');
     $this->assertRaw($field_name . '[1][display]', 'Second file appears as expected.');
   }
diff --git a/core/modules/file/tests/src/Functional/FilePrivateTest.php b/core/modules/file/tests/src/Functional/FilePrivateTest.php
index 1e335eaf43..82cae97540 100644
--- a/core/modules/file/tests/src/Functional/FilePrivateTest.php
+++ b/core/modules/file/tests/src/Functional/FilePrivateTest.php
@@ -6,6 +6,7 @@
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\file\Entity\File;
 use Drupal\node\Entity\NodeType;
+use Drupal\Tests\Traits\DrupalPostFormHack;
 use Drupal\user\RoleInterface;
 
 /**
@@ -15,6 +16,8 @@
  */
 class FilePrivateTest extends FileFieldTestBase {
 
+  use DrupalPostFormHack;
+
   /**
    * Modules to enable.
    *
@@ -80,22 +83,19 @@ public function testPrivateFile() {
     $this->drupalPostForm('node/add/' . $type_name, $edit, t('Save'));
     $new_node = $this->drupalGetNodeByTitle($edit['title[0][value]']);
     $edit[$field_name . '[0][fids]'] = $node_file->id();
-    $this->drupalPostForm('node/' . $new_node->id() . '/edit', $edit, t('Save'));
-    // Make sure the form submit failed - we stayed on the edit form.
-    $this->assertUrl('node/' . $new_node->id() . '/edit');
+    $this->drupalPostFormHack('node/' . $new_node->id() . '/edit', $edit, t('Save'));
     // Check that we got the expected constraint form error.
     $constraint = new ReferenceAccessConstraint();
-    $this->assertRaw(SafeMarkup::format($constraint->message, ['%type' => 'file', '%id' => $node_file->id()]));
+    $this->assertContains(SafeMarkup::format($constraint->message, ['%type' => 'file', '%id' => $node_file->id()])->jsonSerialize(), (string) $this->postResponse->getBody());
     // Attempt to reuse the existing file when creating a new node, and confirm
     // that access is still denied.
     $edit = [];
     $edit['title[0][value]'] = $this->randomMachineName();
     $edit[$field_name . '[0][fids]'] = $node_file->id();
-    $this->drupalPostForm('node/add/' . $type_name, $edit, t('Save'));
+    $this->drupalPostFormHack('node/add/' . $type_name, $edit, t('Save'));
     $new_node = $this->drupalGetNodeByTitle($edit['title[0][value]']);
     $this->assertTrue(empty($new_node), 'Node was not created.');
-    $this->assertUrl('node/add/' . $type_name);
-    $this->assertRaw(SafeMarkup::format($constraint->message, ['%type' => 'file', '%id' => $node_file->id()]));
+    $this->assertContains(SafeMarkup::format($constraint->message, ['%type' => 'file', '%id' => $node_file->id()])->jsonSerialize(), (string) $this->postResponse->getBody());
 
     // Now make file_test_file_download() return everything.
     \Drupal::state()->set('file_test.allow_all', TRUE);
diff --git a/core/modules/file/tests/src/Functional/SaveUploadFormTest.php b/core/modules/file/tests/src/Functional/SaveUploadFormTest.php
index 6ac441770e..ab178707f5 100644
--- a/core/modules/file/tests/src/Functional/SaveUploadFormTest.php
+++ b/core/modules/file/tests/src/Functional/SaveUploadFormTest.php
@@ -361,7 +361,7 @@ public function testDrupalMovingUploadedFileError() {
     \Drupal::state()->set('file_test.disable_error_collection', TRUE);
     $this->drupalPostFormHack('file-test/save_upload_from_form_test', $edit, t('Submit'));
     $this->assertContains(t('File upload error. Could not move uploaded file.')->render(), (string) $this->postResponse->getBody(), 'Found the failure message.');
-    $this->assertContains(t('Epic upload FAIL!')->render(), $this->postResponse->getBody(), 'Found the failure message.');
+    $this->assertContains(t('Epic upload FAIL!')->render(), (string) $this->postResponse->getBody(), 'Found the failure message.');
 
     // Uploading failed. Now check the log.
     $this->drupalGet('admin/reports/dblog');