Upload-field not migrated if not explicitly enabled on D6.

Problem/Motivation

On a D6 site with the upload module enabled, file attachments are considered enabled by default if the upload_[node_type] is not set. Ran into an issue with a custom D6 to D8 migration, where one content type had file attachments enabled on the D6 instance, but the upload field was missing after migration on the D8 site (two other content types with file attachments enabled did have the upload-field on D8 after migration.)

Relevant code snippet in D6's upload module:

function upload_form_alter(&$form, $form_state, $form_id) {
  if ($form_id == 'node_type_form' && isset($form['identity']['type'])) {
    $form['workflow']['upload'] = array(
      '#type' => 'radios',
      '#title' => t('Attachments'),
      '#default_value' => variable_get('upload_'. $form['#node_type']->type, 1),
      '#options' => array(t('Disabled'), t('Enabled')),
    );
  }

  if (isset($form['type']) && isset($form['#node'])) {
    $node = $form['#node'];
    if ($form['type']['#value'] .'_node_form' == $form_id && variable_get("upload_$node->type", TRUE)) {
      // Attachments fieldset
      $form['attachments'] = array(
...

Proposed resolution

Since the "File attachments" field(set) is showing up on the original D6 site when the "upload_[type]" variable is not explicitly set, that content type should be considered as having file attachments enabled. Migrate the upload-field for all content types where it is either explicitly enabled, or where the variable is not set.

Remaining tasks

- Review
- Commit

User interface changes

None

API changes

None

Data model changes

None