Merge pull request cakephp#2330 from cakephp/issue-2326

Add quick example of using uploadedFile rule.

en/core-libraries/validation.rst

@@ -240,10 +240,15 @@ not a particular rule should be applied::
     ]);
 
 The above example will make the rule for 'picture' optional depending on whether
-the value for ``show_profile_picture`` is empty.
+the value for ``show_profile_picture`` is empty. You could also use the
+``uploadedFile`` validation rule to create optional file upload inputs::
 
-The same can be done for the ``allowEmpty()`` and ``notEmpty`` validation method.
-Both take a callable function as the last argument, which determines whether or
+    $validator->add('picture', 'file', [
+        'rule' => ['uploadedFile', ['optional' => true]],
+    ]);
+
+The ``allowEmpty()`` and ``notEmpty()`` methods will also accept a callback
+function as their last argument. If present, the callback determines whether or
 not the rule should be applied. For example, a field can be sometimes allowed
 to be empty::