minor #27183 Fixed return type (tarlepp)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #27183).

Discussion
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Fixed return type

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | no    <!-- please add some, will be required by reviewers -->
| Fixed tickets |    <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

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Note that tests that are failing are not related to this component.

Commits
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5539f9d6c8 Fixed return type

src/Symfony/Component/HttpFoundation/Request.php

@@ -1322,7 +1322,7 @@ class Request
      *
      * @param string $format The format
      *
-     * @return string The associated mime type (null if not found)
+     * @return string|null The associated mime type (null if not found)
      */
     public function getMimeType($format)
     {