plupload implementation within Drupal with validators.

Drupal: Implementing Plupload file save

/**
 * PLUPLOAD example form to showcase plupload.
 */
function MYMODULE_some_cool_form() {
  $form['file_upload'] = array(
    '#type' => 'plupload',
    '#title' => t('Upload Files'),
    '#required' => TRUE,
    '#description' => t('Allowed file extensions: doc docx pdf tif tiff xls xlsx'),
    '#upload_validators' => array(
      'file_validate_extensions' => array('doc docx pdf tif tiff xls xlsx'),
      'mymodule_validate_file_extensions' => array('doc docx pdf tif tiff xls xlsx'),
    ),
    '#plupload_settings' => array(
      'runtimes' => 'html5',
      'drop_element' => FALSE,
      'chunk_size' => '1mb',
      'multi_selection' => TRUE,
    ),
    '#event_callbacks' => array(
      'FilesAdded' => 'Drupal.myModule.FilesAdded',
      'UploadComplete' => 'Drupal.behaviors.myModule.UploadComplete',
      'FilesRemoved' => 'Drupal.behaviors.myModule.FilesRemoved',
    ),
  );
}

/**
 * Custom file validator.
 */
function mymodule_validate_file_extensions($file, $allowed_extensions) {
  $error = file_validate_extensions($file, $allowed_types);

  return $error;
}

/**
 * Submit handler for MYMODULE_some_cool_form().
 */
function MYMODULE_some_cool_form_submit($form, &$form_state) {
  $uploaded_files = ['file_upload'];
  $scheme = 'private://';

  foreach ($uploaded_files as $uploaded_file) {
    if ($uploaded_file['status'] == 'done') {
      $source = $uploaded_file['tmppath'];
      $destination = file_stream_wrapper_uri_normalize($scheme . $uploaded_file['name']);
      $destination = file_unmanaged_move($source, $destination, FILE_EXISTS_REPLACE);

      $file = plupload_file_uri_to_object($destination);
      $file->status = FILE_STATUS_PERMANENT;
      file_save($file);
    }
  }
}

-------- In Javascript ----------
/**
 * Restricting uploaded files to X number of files (maxFiles).
 */
 !(function ($) {
 'use strict';
 
  Drupal.behaviors.myModule = {
    FilesAdded: function(up, addedFiles) {
      var addButton = $('#' + up.settings.browse_button);
      var numFiles = up.files.length;
      var maxFiles = 1;

      // We can only upload 1 file at a time to each order.
      if (numFiles > maxFiles) {
        alert('No more than ' + maxFiles + ' file(s) can be uploaded');

        // Remove the newly added file(s).
        up.splice(maxFiles);
      }
      if (numFiles === maxFiles) {
        // Disable uploading and also hide the button.
        up.trigger("DisableBrowse", true);
        addButton.hide();
      }

    },
    FilesRemoved: function(up, removedFiles) {
      var numFiles = up.files.length;
      var addButton = $('#' + up.settings.browse_button);
      var maxFiles = 1;

      if (numFiles < maxFiles) {
        up.trigger("DisableBrowse", false);
        addButton.show();
      }
    }
    // Other events documented here: http://www.plupload.com/example_events.php#source
  }
})(jQuery);

Cannot figure out how to make plupload respect the file extension list

Solved with custom file validator