How to save a file using file_save_upload in multistep AJAX form

Question

How can I save a file using file_save_upload in a AJAX multistep form? This works for a simple form, but I'm not sure where I'm going wrong in the multistep. Any help is very much appreciated!

File save works in simple form:

function xmlimport_form_submit($form, &$form_state){
$validators = array('file_validate_extensions' => array());
$file = file_save_upload('fid', $validators,'public://',FILE_EXISTS_RENAME);
 }

But doesn't seem to work in multistep. The error I get in my block says, "No file chosen" when I click Next Step on my form. AJAX form taken from Form API Example module.

function xmlimport_wizard($form, &$form_state) {

  $form['#prefix'] = '<div id="wizard-form-wrapper">';
  $form['#suffix'] = '</div>';
  $form['#tree'] = TRUE; // We want to deal with hierarchical form values.

  $step = empty($form_state['storage']['step']) ? 1 : $form_state['storage']['step'];
  $form_state['storage']['step'] = $step;

  switch ($step) {
    case 1:
      $form['step1'] = array(
        '#type' => 'fieldset',
        '#title' => t('Step 1: Log RFI from XML'),
      );
      $form['step1']['fid'] = array(
        '#type' => 'file',
        '#title' => t('Your XML file'),
        '#default_value' => empty($form_state['values']['step1']['fid']) ? '' : $form_state['values']['step1']['fid'],
        '#required' => TRUE,
      );
      break;

    case 2:
      $form['step2'] = array(
        '#type' => 'fieldset',
        '#title' => t('Step 2: Street address info'),
      );
      $form['step2']['project'] = array(
        '#type' => 'textfield',
        '#title' => t('Project Name'),
        '#default_value' => empty($form_state['values']['step2']['project']) ? '' : $form_state['values']['step2']['project'],
        '#required' => TRUE,
        '#size' => 23,
      );
      break;
  }
  if ($step == 2) {
    $form['submit'] = array(
      '#type' => 'submit',
      '#value' => t("Submit your information"),
    );
  }
  if ($step < 2) {
    $form['next'] = array(
      '#type' => 'submit',
      '#value' => t('Next step'),
      '#ajax' => array(
        'wrapper' => 'wizard-form-wrapper',
        'callback' => 'xmlimport_wizard_callback',
      ),
    );
  }
  if ($step > 1) {
    $form['prev'] = array(
      '#type' => 'submit',
      '#value' => t("Previous step"),

      // Since all info will be discarded, don't validate on 'prev'.
      '#limit_validation_errors' => array(),
      // #submit is required to use #limit_validation_errors
      '#submit' => array('xmlimport_wizard_submit'),
      '#ajax' => array(
        'wrapper' => 'wizard-form-wrapper',
        'callback' => 'xmlimport_wizard_callback',
      ),
    );
  }
  return $form;
}

function xmlimport_wizard_callback($form, $form_state) {
    $validators = array('file_validate_extensions' => array());
    $file = file_save_upload('fid', $validators,'public://',FILE_EXISTS_RENAME);

  return $form;
}

/**
 * Submit function for xmlimport_wizard.
 *
 * In AJAX this is only submitted when the final submit button is clicked,
 * but in the non-javascript situation, it is submitted with every
 * button click.
 */
function xmlimport_wizard_submit($form, &$form_state) {

  // Save away the current information.
  $current_step = 'step' . $form_state['storage']['step'];
  if (!empty($form_state['values'][$current_step])) {
    $form_state['storage']['values'][$current_step] = $form_state['values'][$current_step];
  }

  // Increment or decrement the step as needed. Recover values if they exist.
  if ($form_state['triggering_element']['#value'] == t('Next step')) {
    $form_state['storage']['step']++;
    // If values have already been entered for this step, recover them from
    // $form_state['storage'] to pre-populate them.
    $step_name = 'step' . $form_state['storage']['step'];
    if (!empty($form_state['storage']['values'][$step_name])) {
      $form_state['values'][$step_name] = $form_state['storage']['values'][$step_name];
    }
  }
  if ($form_state['triggering_element']['#value'] == t('Previous step')) {
    $form_state['storage']['step']--;
    // Recover our values from $form_state['storage'] to pre-populate them.
    $step_name = 'step' . $form_state['storage']['step'];
    $form_state['values'][$step_name] = $form_state['storage']['values'][$step_name];
  }

  // If they're done, submit.
  if ($form_state['triggering_element']['#value'] == t('Submit your information')) {
    $value_message = t('Your information has been submitted:') . ' ';
    foreach ($form_state['storage']['values'] as $step => $values) {
      $value_message .= "$step: ";
      foreach ($values as $key => $value) {
        $value_message .= "$key=$value, ";
      }
    }
    drupal_set_message($value_message);
    $form_state['rebuild'] = FALSE;
    return;
  }

  // Otherwise, we still have work to do.
  $form_state['rebuild'] = TRUE;
}

Answer 1

have you tried instead of using '#type' => 'file' for your file upload field to use the '#type' => 'managed_file' ? it's easier to validate and it's ajax enabled by default. You can read more about it here

Answer 2

You can try this!!

$form['--Name--'] = array(

         '#type' => 'managed_file',

        '#name' => 'files[]',

        '#title' => t('Upload some multifile'),

        '#description' => t('Allow to upload jpg, jpeg and png, 5MB Max Size'),

        '#attributes' => array('multiple' => 'multiple'),

  );