title: Symfony: form_enctype deprecated
body: <p>Currently I have following code in symfony 2 and I want to upgrade it to symfony 3. I get this deprecation warning to use <code>form_start</code> instead of <code>form_enctype</code>. How can I change the following code with <code>form_start</code>?</p>
  
  <p><strong>View:</strong></p>
  
  <pre><code>&lt;form method="post" {{ form_enctype(decryptionForm) }}&gt;
                                  {{ form_widget(decryptionForm) }}
                                  {{ submit_widget }}
                              &lt;/form&gt;
  </code></pre>
  
  <p><strong>Controller</strong></p>
  
  <pre><code>$form = new DecryptionForm();
          $this-&gt;setContext(array(
              'decryptionForm' =&gt; $form-&gt;createView()
          ));
  </code></pre>
  
  <p><strong>Form</strong></p>
  
  <pre><code>$builder = $this-&gt;getFormBuilder();
          $form = $builder
              -&gt;add(self::FORM_KEY_SAMPLECASE, 'hidden', array())
              -&gt;add(self::FORM_KEY_DECRKEYFILE, 'file', array(
                  'constraints' =&gt; array(new NotBlank()),
                  'label' =&gt; "private_key"
              ))
              -&gt;add("Submit", 'submit')
              -&gt;getForm();
  </code></pre>