minor #9179 uploading example: explaining the need for md5 (MacDada, javiereguiluz)

This PR was submitted for the 4.0 branch but it was merged into the 2.7 branch instead (closes #9179).

Discussion
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uploading example: explaining the need for md5

Plus extracted function with a name that makes the comment unnecessary.

Commits
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9321577 Reword a code comment
6185887 uploading example: explaining the need for md5

Author: javiereguiluz

controller/upload_file.rst

@@ -136,8 +136,7 @@ Finally, you need to update the code of the controller that handles the form::
                 /** @var Symfony\Component\HttpFoundation\File\UploadedFile $file */
                 $file = $product->getBrochure();
 
-                // Generate a unique name for the file before saving it
-                $fileName = md5(uniqid()).'.'.$file->guessExtension();
+                $fileName = $this->generateUniqueFileName().'.'.$file->guessExtension();
 
                 // Move the file to the directory where brochures are stored
                 $file->move(
@@ -158,6 +157,16 @@ Finally, you need to update the code of the controller that handles the form::
                 'form' => $form->createView(),
             ));
         }
+        
+        /**
+         * @return string
+         */
+        private function generateUniqueFileName()
+        {
+            // md5() reduces the similarity of the file names generated by
+            // uniqid(), which is based on timestamps
+            return md5(uniqid());
+        }
     }
 
 Now, create the ``brochures_directory`` parameter that was used in the