minor #20893 [Validator] Highlight extensions instead of mimeTypes for File constraint (benito103e)

This PR was submitted for the 7.2 branch but it was merged into the 6.4 branch instead.

Discussion
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[Validator] Highlight `extensions` instead of `mimeTypes` for `File` constraint

You should always use the extensions option instead of mimeTypes except if you explicitly don't want to check that the extension of the file is consistent with its content (this can be a security issue).

By default, the extensions option also checks the media type of the file.

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Commits
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3474a09 Highligh 'extensions' instead of 'mimeTypes' for File constraint

Author: xabbuh

controller/upload_file.rst

@@ -77,11 +77,8 @@ so Symfony doesn't try to get/set its value from the related entity::
                     'constraints' => [
                         new File([
                             'maxSize' => '1024k',
-                            'mimeTypes' => [
-                                'application/pdf',
-                                'application/x-pdf',
-                            ],
-                            'mimeTypesMessage' => 'Please upload a valid PDF document',
+                            'extensions' => ['pdf'],
+                            'extensionsMessage' => 'Please upload a valid PDF document',
                         ])
                     ],
                 ])