minor #8081 File example update (runawaycoin)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #8081).

Discussion
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File example update

Update code example to highlight where the $file comes from in second example

Commits
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e7fb1a1 File example update

Author: xabbuh

reference/forms/types/file.rst

@@ -50,7 +50,8 @@ be used to move the ``attachment`` file to a permanent location::
         if ($form->isSubmitted() && $form->isValid()) {
             $someNewFilename = ...
 
-            $form['attachment']->getData()->move($dir, $someNewFilename);
+            $file = $form['attachment']->getData();
+            $file->move($dir, $someNewFilename);
 
             // ...
         }