minor #18722 [Form] Fix File Upload Example (nacholibre)

This PR was submitted for the 5.4 branch but it was merged into the 6.4 branch instead.

Discussion
----------

[Form] Fix File Upload Example

Currently the example suggests to execute this code when doing edit
```php
$product->setBrochureFilename(
    new File($this->getParameter('brochures_directory').'/'.$product->getBrochureFilename())
);
```

this is wrong in couple of ways

First the `setBrochureFilename` method expects string and not a `File` instance. No error is thrown because the `File` instance can be casted to `string` and will return the full path, which is not what's needed. In the database field you need to store the filename only, and not the full path.

Second the form expects `File` instance and not the entity. This is why the form data needs to be changed and not the entity.

In the 4.4 branch it's the same.

Commits
-------

e2fb274 Fix File Upload Example

Author: javiereguiluz

controller/upload_file.rst

@@ -206,9 +206,13 @@ You can use the following code to link to the PDF brochure of a product:
         use Symfony\Component\HttpFoundation\File\File;
         // ...
 
-        $product->setBrochureFilename(
-            new File($brochuresDirectory.DIRECTORY_SEPARATOR.$product->getBrochureFilename())
-        );
+        if (!$form->isSubmitted() && $product->getBrochureFilename()) {
+            $form->setData(['brochure' => new File($this->getParameter('brochures_directory').DIRECTORY_SEPARATOR.$product->getBrochureFilename())]);
+        }
+
+        if ($form->isSubmitted() && $form->isValid()) {
+            // ...
+        }
 
 Creating an Uploader Service
 ----------------------------