Add autoupload property after page load using plupload
Drupal's plupload module does not support '#autoupload'. Or at least that's what I experienced. I wrote my own behavior for this:

(function($) {

Drupal.plupload = Drupal.plupload || {};

/**
 * Attaches the Plupload behavior to each Plupload form element.
 */
Drupal.behaviors.mymodule_plupload = {
  attach: function (context, settings) {
    $(".plupload-element", context).once('mymodule-plupload-init', function () {
      var uploader = $(this).pluploadQueue();
      if(uploader !== undefined) {
        uploader.bind('FilesAdded', function(up, files) {
          $('.form-submit').prop( "disabled", true );
          uploader.start();
        });
        uploader.bind('UploadComplete', function() {
          if (uploader.files.length == (uploader.total.uploaded + uploader.total.failed)) {
            // If nothing is being uploaded right now:
            $('.form-submit').prop( "disabled", false );
          }
        });
      }
    });
  }
};

})(jQuery);


I based it on codes and hints from the Internet, but I can't recall exact sources. I'll be happy to fix that and add reference links if someone recognizes parts of this.