This PR was merged into the 5.4 branch.

Discussion
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[Form] Fix merging form data and files (ter)

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        | Fix #52318
| License       | MIT

Adapted from a patch provided by `@jan`-pintr

The original was like below but this one looks more appropriate to me.

<details>

```diff
--- a/src/Symfony/Component/Form/Util/FormUtil.php
+++ b/src/Symfony/Component/Form/Util/FormUtil.php
@@ -47,8 +47,12 @@ public static function isEmpty(mixed $data): bool
     public static function mergeParamsAndFiles(array $params, array $files): array
     {
         if (array_is_list($files)) {
-            foreach ($files as $value) {
-                $params[] = $value;
+            foreach ($files as $key => $value) {
+                if (is_array($params[$key])) {
+                    $params[$key] = array_merge($params[$key], $value);
+                } else {
+                    $params[] = $value;
+                }
             }

             return $params;
@@ -61,6 +65,6 @@ public static function mergeParamsAndFiles(array $params, array $files): array
             }
         }

-        return array_replace($params, $files);
+        return array_merge($params, $files);
     }```

</details>

Commits
-------

9facb2f [Form] Fix merging form data and files (ter)