[Form] tweaked the error message when a form with a file upload does not have the enctype attribute correctly configured

src/Symfony/Component/Form/Extension/Core/DataTransformer/FileToArrayTransformer.php

@@ -64,11 +64,11 @@ class FileToArrayTransformer implements DataTransformerInterface
         }
 
         if (!array_key_exists('file', $array)) {
-            throw new TransformationFailedException('The key "file" is missing');
+            throw new TransformationFailedException('The key "file" is missing.');
         }
 
         if (!empty($array['file']) && !$array['file'] instanceof File) {
-            throw new TransformationFailedException('The key "file" should be empty or instance of File');
+            throw new TransformationFailedException('The key "file" should be empty or instance of File (Have you set the "enctype" attribute of the form tag to "multipart/form-data"?).');
         }
 
         return $array['file'];