diff --git a/src/Symfony/Component/Form/Extension/Core/DataTransformer/FileToArrayTransformer.php b/src/Symfony/Component/Form/Extension/Core/DataTransformer/FileToArrayTransformer.php
index 3a16ed58fb..58c0e0f3b2 100644
--- a/src/Symfony/Component/Form/Extension/Core/DataTransformer/FileToArrayTransformer.php
+++ b/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'];