diff --git a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php
index 8c5dd76351..98ac211a44 100644
--- a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php
+++ b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php
@@ -1767,7 +1767,7 @@ class FrameworkExtension extends Extension
         }
 
         $loader->load('mailer.xml');
-        $container->getDefinition('mailer.transport')->setArgument(0, $config['dsn']);
+        $container->getDefinition('mailer.default_transport')->setArgument(0, $config['dsn']);
     }
 
     /**
diff --git a/src/Symfony/Bundle/FrameworkBundle/Resources/config/mailer.xml b/src/Symfony/Bundle/FrameworkBundle/Resources/config/mailer.xml
index d39899c0a9..1f567a6c93 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Resources/config/mailer.xml
+++ b/src/Symfony/Bundle/FrameworkBundle/Resources/config/mailer.xml
@@ -6,23 +6,23 @@
 
     <services>
         <service id="mailer.mailer" class="Symfony\Component\Mailer\Mailer">
-            <argument type="service" id="mailer.transport" />
+            <argument type="service" id="mailer.default_transport" />
             <argument type="service" id="messenger.default_bus" on-invalid="ignore" />
         </service>
         <service id="mailer" alias="mailer.mailer" />
         <service id="Symfony\Component\Mailer\MailerInterface" alias="mailer.mailer" />
 
-        <service id="mailer.transport" class="Symfony\Component\Mailer\Transport\TransportInterface">
+        <service id="mailer.default_transport" class="Symfony\Component\Mailer\Transport\TransportInterface">
             <factory class="Symfony\Component\Mailer\Transport" method="fromDsn" />
             <argument /> <!-- env(MAILER_DSN) -->
             <argument type="service" id="event_dispatcher" />
             <argument type="service" id="http_client" on-invalid="ignore" />
             <argument type="service" id="logger" on-invalid="ignore" />
         </service>
-        <service id="Symfony\Component\Mailer\Transport\TransportInterface" alias="mailer.transport" />
+        <service id="Symfony\Component\Mailer\Transport\TransportInterface" alias="mailer.default_transport" />
 
         <service id="mailer.messenger.message_handler" class="Symfony\Component\Mailer\Messenger\MessageHandler">
-            <argument type="service" id="mailer.transport" />
+            <argument type="service" id="mailer.default_transport" />
             <tag name="messenger.message_handler" />
         </service>
     </services>
diff --git a/src/Symfony/Component/Form/Extension/Core/Type/FileType.php b/src/Symfony/Component/Form/Extension/Core/Type/FileType.php
index 4dc71db4f2..6c62c2fafd 100644
--- a/src/Symfony/Component/Form/Extension/Core/Type/FileType.php
+++ b/src/Symfony/Component/Form/Extension/Core/Type/FileType.php
@@ -35,8 +35,14 @@ class FileType extends AbstractType
 
     private $translator;
 
-    public function __construct(TranslatorInterface $translator = null)
+    /**
+     * @param TranslatorInterface|null $translator
+     */
+    public function __construct($translator = null)
     {
+        if (null !== $translator && !$translator instanceof LegacyTranslatorInterface && !$translator instanceof TranslatorInterface) {
+            throw new \TypeError(sprintf('Argument 1 passed to %s() must be an instance of %s, %s given.', __METHOD__, TranslatorInterface::class, \is_object($translator) ? \get_class($translator) : \gettype($translator)));
+        }
         $this->translator = $translator;
     }
 
diff --git a/src/Symfony/Component/Serializer/Normalizer/DataUriNormalizer.php b/src/Symfony/Component/Serializer/Normalizer/DataUriNormalizer.php
index 3ab7bd45f8..a6ca7223b0 100644
--- a/src/Symfony/Component/Serializer/Normalizer/DataUriNormalizer.php
+++ b/src/Symfony/Component/Serializer/Normalizer/DataUriNormalizer.php
@@ -34,12 +34,12 @@ class DataUriNormalizer implements NormalizerInterface, DenormalizerInterface, C
     ];
 
     /**
-     * @var MimeTypeGuesserInterface
+     * @var MimeTypeGuesserInterface|null
      */
     private $mimeTypeGuesser;
 
     /**
-     * @param MimeTypeGuesserInterface
+     * @param MimeTypeGuesserInterface|null $mimeTypeGuesser
      */
     public function __construct($mimeTypeGuesser = null)
     {
@@ -48,8 +48,8 @@ class DataUriNormalizer implements NormalizerInterface, DenormalizerInterface, C
         } elseif (null === $mimeTypeGuesser) {
             if (class_exists(MimeTypes::class)) {
                 $mimeTypeGuesser = MimeTypes::getDefault();
-            } else {
-                @trigger_error(sprintf('Passing null to "%s()" without symfony/mime installed is deprecated since Symfony 4.3, install symfony/mime.', __METHOD__), E_USER_DEPRECATED);
+            } elseif (class_exists(MimeTypeGuesser::class)) {
+                @trigger_error(sprintf('Passing null to "%s()" to use a default MIME type guesser without Symfony Mime installed is deprecated since Symfony 4.3. Try running "composer require symfony/mime".', __METHOD__), E_USER_DEPRECATED);
                 $mimeTypeGuesser = MimeTypeGuesser::getInstance();
             }
         } elseif (!$mimeTypeGuesser instanceof MimeTypes) {
@@ -156,7 +156,9 @@ class DataUriNormalizer implements NormalizerInterface, DenormalizerInterface, C
 
         if ($this->mimeTypeGuesser instanceof DeprecatedMimeTypeGuesserInterface && $mimeType = $this->mimeTypeGuesser->guess($object->getPathname())) {
             return $mimeType;
-        } elseif ($this->mimeTypeGuesser && $mimeType = $this->mimeTypeGuesser->guessMimeType($object->getPathname())) {
+        }
+
+        if ($this->mimeTypeGuesser && $mimeType = $this->mimeTypeGuesser->guessMimeType($object->getPathname())) {
             return $mimeType;
         }
 
diff --git a/src/Symfony/Component/Serializer/composer.json b/src/Symfony/Component/Serializer/composer.json
index 626bf238d8..acf2d170c1 100644
--- a/src/Symfony/Component/Serializer/composer.json
+++ b/src/Symfony/Component/Serializer/composer.json
@@ -45,7 +45,7 @@
         "symfony/yaml": "For using the default YAML mapping loader.",
         "symfony/config": "For using the XML mapping loader.",
         "symfony/property-access": "For using the ObjectNormalizer.",
-        "symfony/http-foundation": "To use the DataUriNormalizer.",
+        "symfony/http-foundation": "For using a MIME type guesser within the DataUriNormalizer.",
         "doctrine/annotations": "For using the annotation mapping. You will also need doctrine/cache.",
         "doctrine/cache": "For using the default cached annotation reader and metadata cache."
     },