diff --git a/core/core.services.yml b/core/core.services.yml
index cbe9d84..97c2dcb 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -282,7 +282,7 @@ services:
     arguments: ['@stream_wrapper_manager', '@settings', '@logger.channel.file']
   form_builder:
     class: Drupal\Core\Form\FormBuilder
-    arguments: ['@form_validator', '@form_submitter', '@form_cache', '@module_handler', '@event_dispatcher', '@request_stack', '@class_resolver', '@element_info', '@theme.manager', '@string_translation', '@?csrf_token']
+    arguments: ['@form_validator', '@form_submitter', '@form_cache', '@module_handler', '@event_dispatcher', '@request_stack', '@class_resolver', '@element_info', '@theme.manager', '@?csrf_token']
   form_validator:
     class: Drupal\Core\Form\FormValidator
     arguments: ['@request_stack', '@string_translation', '@csrf_token', '@logger.channel.form', '@form_error_handler']
@@ -821,7 +821,7 @@ services:
       - { name: event_subscriber }
   form_ajax_subscriber:
     class: Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber
-    arguments: ['@form_ajax_response_builder', '@renderer']
+    arguments: ['@form_ajax_response_builder', '@renderer', '@string_translation']
     tags:
       - { name: event_subscriber }
   route_enhancer.lazy_collector:
diff --git a/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
index 73fe812..09c68cd 100644
--- a/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
+++ b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
@@ -10,14 +10,16 @@
 use Drupal\Core\Ajax\AjaxResponse;
 use Drupal\Core\Ajax\ReplaceCommand;
 use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
+use Drupal\Core\Form\Exception\BrokenPostRequestException;
 use Drupal\Core\Form\FormAjaxException;
 use Drupal\Core\Form\FormAjaxResponseBuilderInterface;
 use Drupal\Core\Form\FormBuilderInterface;
 use Drupal\Core\Render\RendererInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\StringTranslation\TranslationInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
-use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
 use Symfony\Component\HttpKernel\KernelEvents;
 
 /**
@@ -25,6 +27,8 @@
  */
 class FormAjaxSubscriber implements EventSubscriberInterface {
 
+  use StringTranslationTrait;
+
   /**
    * The form AJAX response builder.
    *
@@ -46,10 +50,13 @@ class FormAjaxSubscriber implements EventSubscriberInterface {
    *   The form AJAX response builder.
    * @param \Drupal\Core\Render\RendererInterface $renderer
    *   The renderer service.
+   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
+   *   The string translation.
    */
-  public function __construct(FormAjaxResponseBuilderInterface $form_ajax_response_builder, RendererInterface $renderer) {
+  public function __construct(FormAjaxResponseBuilderInterface $form_ajax_response_builder, RendererInterface $renderer, TranslationInterface $string_translation) {
     $this->formAjaxResponseBuilder = $form_ajax_response_builder;
     $this->renderer = $renderer;
+    $this->stringTranslation = $string_translation;
   }
 
   /**
@@ -83,8 +90,8 @@ public function onException(GetResponseForExceptionEvent $event) {
 
     // Render a nice error message in case we have a file upload which exceeds
     // the configured upload limit.
-    if ($exception instanceof BadRequestHttpException && $request->query->has(FormBuilderInterface::AJAX_FORM_REQUEST)) {
-      $this->drupalSetMessage($exception->getMessage(), 'error');
+    if ($exception instanceof BrokenPostRequestException && $request->query->has(FormBuilderInterface::AJAX_FORM_REQUEST)) {
+      $this->drupalSetMessage($this->t('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (@size) that this server supports.', ['@size' => $this->formatSize($exception->getSize())]), 'error');
       $response = new AjaxResponse();
       $status_messages = ['#type' => 'status_messages'];
       $response->addCommand(new ReplaceCommand(NULL, $this->renderer->renderRoot($status_messages)));
@@ -141,6 +148,16 @@ protected function getFormAjaxException(\Exception $e) {
   }
 
   /**
+   * Wraps format_size()
+   *
+   * @return string
+   *   The formatted size.
+   */
+  protected function formatSize($size) {
+    return format_size($size);
+  }
+
+  /**
    * {@inheritdoc}
    */
   public static function getSubscribedEvents() {
diff --git a/core/lib/Drupal/Core/Form/Exception/BrokenPostRequestException.php b/core/lib/Drupal/Core/Form/Exception/BrokenPostRequestException.php
new file mode 100644
index 0000000..c6cbba8
--- /dev/null
+++ b/core/lib/Drupal/Core/Form/Exception/BrokenPostRequestException.php
@@ -0,0 +1,54 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Form\BrokenPostRequestException.
+ */
+
+namespace Drupal\Core\Form\Exception;
+
+use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
+
+/**
+ * Defines an exception used, when the POST HTTP body is broken.
+ */
+class BrokenPostRequestException extends BadRequestHttpException {
+
+  /**
+   * The maximum upload size.
+   *
+   * @var string
+   */
+  protected $size;
+
+  /**
+   * Constructs a new BrokenPostRequestException.
+   *
+   * @param string $max_upload_size
+   *   The size of the maximum upload size.
+   * @param string $message
+   *   The internal exception message.
+   * @param \Exception $previous
+   *   The previous exception.
+   * @param int $code
+   *   The internal exception code.
+   */
+  public function __construct($max_upload_size, $message = NULL, \Exception $previous = NULL, $code = 0) {
+    parent::__construct($message, $previous, $code);
+
+    $this->size = $max_upload_size;
+  }
+
+  /**
+   * Returns the maximum upload size.
+   *
+   * @return string
+   *   A translated string representation of the size of the file size limit
+   *   based on the PHP upload_max_filesize and post_max_size.
+   */
+  public function getSize() {
+    return $this->size;
+  }
+
+
+}
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 733618e..10b2153 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -16,6 +16,7 @@
 use Drupal\Core\DependencyInjection\ClassResolverInterface;
 use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
 use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Form\Exception\BrokenPostRequestException;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Render\ElementInfoManagerInterface;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
@@ -24,7 +25,6 @@
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\HttpFoundation\Response;
-use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
 
 /**
  * Provides form building and processing.
@@ -129,12 +129,10 @@ class FormBuilder implements FormBuilderInterface, FormValidatorInterface, FormS
    *   The element info manager.
    * @param \Drupal\Core\Theme\ThemeManagerInterface $theme_manager
    *   The theme manager.
-   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
-   *   The string translation service.
    * @param \Drupal\Core\Access\CsrfTokenGenerator $csrf_token
    *   The CSRF token generator.
    */
-  public function __construct(FormValidatorInterface $form_validator, FormSubmitterInterface $form_submitter, FormCacheInterface $form_cache, ModuleHandlerInterface $module_handler, EventDispatcherInterface $event_dispatcher, RequestStack $request_stack, ClassResolverInterface $class_resolver, ElementInfoManagerInterface $element_info, ThemeManagerInterface $theme_manager, TranslationInterface $string_translation, CsrfTokenGenerator $csrf_token = NULL) {
+  public function __construct(FormValidatorInterface $form_validator, FormSubmitterInterface $form_submitter, FormCacheInterface $form_cache, ModuleHandlerInterface $module_handler, EventDispatcherInterface $event_dispatcher, RequestStack $request_stack, ClassResolverInterface $class_resolver, ElementInfoManagerInterface $element_info, ThemeManagerInterface $theme_manager, CsrfTokenGenerator $csrf_token = NULL) {
     $this->formValidator = $form_validator;
     $this->formSubmitter = $form_submitter;
     $this->formCache = $form_cache;
@@ -145,7 +143,6 @@ public function __construct(FormValidatorInterface $form_validator, FormSubmitte
     $this->elementInfo = $element_info;
     $this->csrfToken = $csrf_token;
     $this->themeManager = $theme_manager;
-    $this->stringTranslation = $string_translation;
   }
 
   /**
@@ -276,7 +273,7 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
     // (post_max_size), the post request is potentially broken. Add some
     // protection against that and at the same time have a nice error message.
     if ($ajax_form_request && !isset($form_state->getUserInput()['form_id'])) {
-      throw new BadRequestHttpException($this->t('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (@size) that this server supports.', ['@size' => $this->getFormattedFileUploadMaxSize()]));
+      throw new BrokenPostRequestException($this->getFileUploadMaxSize());
     }
 
     // After processing the form, if this is an AJAX form request, interrupt
@@ -1167,14 +1164,14 @@ protected function buttonWasClicked($element, FormStateInterface &$form_state) {
   }
 
   /**
-   * Wraps format_size() and file_upload_max_size().
+   * Wraps file_upload_max_size().
    *
    * @return string
    *   A translated string representation of the size of the file size limit
    *   based on the PHP upload_max_filesize and post_max_size.
    */
-  protected function getFormattedFileUploadMaxSize() {
-    return format_size(file_upload_max_size());
+  protected function getFileUploadMaxSize() {
+    return file_upload_max_size();
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Form/EventSubscriber/FormAjaxSubscriberTest.php b/core/tests/Drupal/Tests/Core/Form/EventSubscriber/FormAjaxSubscriberTest.php
index de67c43..390f517 100644
--- a/core/tests/Drupal/Tests/Core/Form/EventSubscriber/FormAjaxSubscriberTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/EventSubscriber/FormAjaxSubscriberTest.php
@@ -9,6 +9,7 @@
 
 use Drupal\Core\Ajax\AjaxResponse;
 use Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber;
+use Drupal\Core\Form\Exception\BrokenPostRequestException;
 use Drupal\Core\Form\FormAjaxException;
 use Drupal\Core\Form\FormBuilderInterface;
 use Drupal\Core\Form\FormState;
@@ -49,6 +50,13 @@ class FormAjaxSubscriberTest extends UnitTestCase {
   protected $renderer;
 
   /**
+   * The mocked string translation.
+   *
+   * @var \Drupal\Core\StringTranslation\TranslationInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $stringTranslation;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUp() {
@@ -57,7 +65,8 @@ protected function setUp() {
     $this->httpKernel = $this->getMock('Symfony\Component\HttpKernel\HttpKernelInterface');
     $this->formAjaxResponseBuilder = $this->getMock('Drupal\Core\Form\FormAjaxResponseBuilderInterface');
     $this->renderer = $this->getMock('\Drupal\Core\Render\RendererInterface');
-    $this->subscriber = new FormAjaxSubscriber($this->formAjaxResponseBuilder, $this->renderer);
+    $this->stringTranslation = $this->getStringTranslationStub();
+    $this->subscriber = new FormAjaxSubscriber($this->formAjaxResponseBuilder, $this->renderer, $this->stringTranslation);
   }
 
   /**
@@ -147,22 +156,26 @@ public function testOnExceptionResponseBuilderException() {
   /**
    * @covers ::onException
    */
-  public function testOnExceptionBadRequest() {
+  public function testOnExceptionBrokenPostRequest() {
     $this->formAjaxResponseBuilder->expects($this->never())
       ->method('buildResponse');
     $this->subscriber = $this->getMockBuilder('\Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber')
-      ->setConstructorArgs([$this->formAjaxResponseBuilder, $this->renderer])
-      ->setMethods(['drupalSetMessage'])
+      ->setConstructorArgs([$this->formAjaxResponseBuilder, $this->renderer, $this->getStringTranslationStub()])
+      ->setMethods(['drupalSetMessage', 'formatSize'])
       ->getMock();
     $this->subscriber->expects($this->once())
       ->method('drupalSetMessage')
       ->willReturn('asdf');
+    $this->subscriber->expects($this->once())
+      ->method('formatSize')
+      ->with(32 * 1e6)
+      ->willReturn('32M');
     $rendered_output = 'the rendered output';
     $this->renderer->expects($this->once())
       ->method('renderRoot')
       ->willReturn($rendered_output);
 
-    $exception = new BadRequestHttpException();
+    $exception = new BrokenPostRequestException(32 * 1e6);
     $request = new Request([FormBuilderInterface::AJAX_FORM_REQUEST => TRUE]);
 
     $event = new GetResponseForExceptionEvent($this->httpKernel, $request, HttpKernelInterface::MASTER_REQUEST, $exception);
diff --git a/core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php b/core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php
index 69c50da..6044570 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php
@@ -429,20 +429,19 @@ public function testFormCacheDeletionUncached() {
   /**
    * @covers ::buildForm
    *
-   * @expectedException \Symfony\Component\HttpKernel\Exception\BadRequestHttpException
-   * @expectedExceptionMessage The uploaded file likely exceeded the maximum file size (32M) that this server supports.
+   * @expectedException \Drupal\Core\Form\Exception\BrokenPostRequestException
    */
   public function testExceededFileSize() {
     $request = new Request([FormBuilderInterface::AJAX_FORM_REQUEST => TRUE]);
     $request_stack = new RequestStack();
     $request_stack->push($request);
     $this->formBuilder = $this->getMockBuilder('\Drupal\Core\Form\FormBuilder')
-      ->setConstructorArgs([$this->formValidator, $this->formSubmitter, $this->formCache, $this->moduleHandler, $this->eventDispatcher, $request_stack, $this->classResolver, $this->elementInfo, $this->themeManager, $this->getStringTranslationStub(), $this->csrfToken])
-      ->setMethods(['getFormattedFileUploadMaxSize'])
+      ->setConstructorArgs([$this->formValidator, $this->formSubmitter, $this->formCache, $this->moduleHandler, $this->eventDispatcher, $request_stack, $this->classResolver, $this->elementInfo, $this->themeManager, $this->csrfToken])
+      ->setMethods(['getFileUploadMaxSize'])
       ->getMock();
     $this->formBuilder->expects($this->once())
-      ->method('getFormattedFileUploadMaxSize')
-      ->willReturn('32M');
+      ->method('getFileUploadMaxSize')
+      ->willReturn(33554432);
 
     $form_arg = $this->getMockForm('test_form_id');
     $form_state = new FormState();
diff --git a/core/tests/Drupal/Tests/Core/Form/FormTestBase.php b/core/tests/Drupal/Tests/Core/Form/FormTestBase.php
index fb48edf..bbda6f8 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormTestBase.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormTestBase.php
@@ -150,13 +150,6 @@
   protected $themeManager;
 
   /**
-   * The string translation service.
-   *
-   * @var \Drupal\Core\StringTranslation\TranslationInterface|\PHPUnit_Framework_MockObject_MockObject
-   */
-  protected $stringTranslation;
-
-  /**
    * {@inheritdoc}
    */
   protected function setUp() {
@@ -200,9 +193,8 @@ protected function setUp() {
       ->setMethods(array('batchGet', 'drupalInstallationAttempted'))
       ->getMock();
     $this->root = dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__))));
-    $this->stringTranslation = $this->getStringTranslationStub();
 
-    $this->formBuilder = new FormBuilder($this->formValidator, $this->formSubmitter, $this->formCache, $this->moduleHandler, $this->eventDispatcher, $this->requestStack, $this->classResolver, $this->elementInfo, $this->themeManager, $this->stringTranslation, $this->csrfToken);
+    $this->formBuilder = new FormBuilder($this->formValidator, $this->formSubmitter, $this->formCache, $this->moduleHandler, $this->eventDispatcher, $this->requestStack, $this->classResolver, $this->elementInfo, $this->themeManager, $this->csrfToken);
   }
 
   /**