title: how to add default value for manged_file in custom configration entity type
body: <p>I have created a custom configuration entity form which have a image field {{ manged_file}} . Here, I have to set its default.</p>
  
  <p>Below is my form element code:</p>
  
  <pre><code>$img = $vajra_entity-&gt;getBgimage();
  
    $form['background']['bgimage'] = [
      '#type'              =&gt; 'managed_file',
      '#title'             =&gt; t('Background Image'),
      '#default_value'     =&gt; $img,
      '#upload_validators' =&gt; array(
        'file_validate_extensions' =&gt; array('gif png jpg jpeg'),
        'file_validate_size'       =&gt; array(25600000),
      ),
      '#upload_location'   =&gt; 'public://Background',
      '#description'       =&gt; $this-&gt;t('Add image for body of a page (gif, png, jpg, jpeg).'),
    ];
  </code></pre>
  
  <p>When I submit form I got following error:</p>
  
  <pre><code>Drupal\Core\Database\DatabaseExceptionWrapper: SQLSTATE[HY000]: General error: 1364 Field 'type' doesn't have a default value: INSERT INTO {file_managed} (uuid, langcode, uid, filename, uri, filemime, filesize, status, created, changed) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9); Array ( [:db_insert_placeholder_0] =&gt; 77433757-4b5c-432b-b7d4-b647e90d40bf [:db_insert_placeholder_1] =&gt; en [:db_insert_placeholder_2] =&gt; 1 [:db_insert_placeholder_3] =&gt; 2015-8-14-drupal-8-hero_0.png [:db_insert_placeholder_4] =&gt; public://Background/2015-8-14-drupal-8-hero_0_0.png [:db_insert_placeholder_5] =&gt; image/png [:db_insert_placeholder_6] =&gt; 6119 [:db_insert_placeholder_7] =&gt; 0 [:db_insert_placeholder_8] =&gt; 1490073466 [:db_insert_placeholder_9] =&gt; 1490073466 ) in Drupal\Core\Entity\Sql\SqlContentEntityStorage-&gt;doSaveFieldItems() (line 843 of /opt/lampp/htdocs/webarch/docroot/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php).
  </code></pre>
  
  <p>Any help will be appreciated :)</p>