// Helper function to wait for appropriate MIME-type headers and
  // then prompt the user with a file picker
  saveHelper: function(linkURL, linkText, dialogTitle, bypassCache, doc) {
    // canonical def in nsURILoader.h
    const NS_ERROR_SAVE_LINK_AS_TIMEOUT = 0x805d0020;

    // an object to proxy the data through to
    // nsIExternalHelperAppService.doContent, which will wait for the
    // appropriate MIME-type headers and then prompt the user with a
    // file picker
    function saveAsListener() {}
    saveAsListener.prototype = {
      extListener: null, 

      onStartRequest: function saveLinkAs_onStartRequest(aRequest, aContext) {

        // if the timer fired, the error status will have been caused by that,
        // and we'll be restarting in onStopRequest, so no reason to notify
        // the user
        if (aRequest.status == NS_ERROR_SAVE_LINK_AS_TIMEOUT)
          return;

        timer.cancel();

        // some other error occured; notify the user...
        if (!Components.isSuccessCode(aRequest.status)) {
          try {
            const sbs = Cc["@mozilla.org/intl/stringbundle;1"].
                        getService(Ci.nsIStringBundleService);
            const bundle = sbs.createBundle(
                    "chrome://mozapps/locale/downloads/downloads.properties");

            const title = bundle.GetStringFromName("downloadErrorAlertTitle");
            const msg = bundle.GetStringFromName("downloadErrorGeneric");

            const promptSvc = Cc["@mozilla.org/embedcomp/prompt-service;1"].
                              getService(Ci.nsIPromptService);
            promptSvc.alert(doc.defaultView, title, msg);
          } catch (ex) {}
          return;
        }

        var extHelperAppSvc = 
          Cc["@mozilla.org/uriloader/external-helper-app-service;1"].
          getService(Ci.nsIExternalHelperAppService);
        var channel = aRequest.QueryInterface(Ci.nsIChannel);
        this.extListener = 
          extHelperAppSvc.doContent(channel.contentType, aRequest, 
                                    doc.defaultView, true);
        this.extListener.onStartRequest(aRequest, aContext);
      }, 

      onStopRequest: function saveLinkAs_onStopRequest(aRequest, aContext, 
                                                       aStatusCode) {
        if (aStatusCode == NS_ERROR_SAVE_LINK_AS_TIMEOUT) {
          // do it the old fashioned way, which will pick the best filename
          // it can without waiting.
          saveURL(linkURL, linkText, dialogTitle, bypassCache, false, doc.documentURIObject);
        }
        if (this.extListener)
          this.extListener.onStopRequest(aRequest, aContext, aStatusCode);
      },

      onDataAvailable: function saveLinkAs_onDataAvailable(aRequest, aContext,
                                                           aInputStream,
                                                           aOffset, aCount) {
        this.extListener.onDataAvailable(aRequest, aContext, aInputStream,
                                         aOffset, aCount);
      }
    }

    function callbacks() {}
    callbacks.prototype = {
      getInterface: function sLA_callbacks_getInterface(aIID) {
        if (aIID.equals(Ci.nsIAuthPrompt) || aIID.equals(Ci.nsIAuthPrompt2)) {
          // If the channel demands authentication prompt, we must cancel it
          // because the save-as-timer would expire and cancel the channel
          // before we get credentials from user.  Both authentication dialog
          // and save as dialog would appear on the screen as we fall back to
          // the old fashioned way after the timeout.
          timer.cancel();
          channel.cancel(NS_ERROR_SAVE_LINK_AS_TIMEOUT);
        }
        throw Cr.NS_ERROR_NO_INTERFACE;
      } 
    }

    // if it we don't have the headers after a short time, the user 
    // won't have received any feedback from their click.  that's bad.  so
    // we give up waiting for the filename. 
    function timerCallback() {}
    timerCallback.prototype = {
      notify: function sLA_timer_notify(aTimer) {
        channel.cancel(NS_ERROR_SAVE_LINK_AS_TIMEOUT);
        return;
      }
    }

    // set up a channel to do the saving
    var ioService = Cc["@mozilla.org/network/io-service;1"].
                    getService(Ci.nsIIOService);
    var channel = ioService.newChannelFromURI(makeURI(linkURL));
    channel.notificationCallbacks = new callbacks();

    let flags = Ci.nsIChannel.LOAD_CALL_CONTENT_SNIFFERS;

    if (bypassCache)
      flags |= Ci.nsIRequest.LOAD_BYPASS_CACHE;

    if (channel instanceof Ci.nsICachingChannel)
      flags |= Ci.nsICachingChannel.LOAD_BYPASS_LOCAL_CACHE_IF_BUSY;

    channel.loadFlags |= flags;

    if (channel instanceof Ci.nsIHttpChannel) {
      channel.referrer = doc.documentURIObject;
      if (channel instanceof Ci.nsIHttpChannelInternal)
        channel.forceAllowThirdPartyCookie = true;
    }

    // fallback to the old way if we don't see the headers quickly 
    var timeToWait = 
      gPrefService.getIntPref("browser.download.saveLinkAsFilenameTimeout");
    var timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
    timer.initWithCallback(new timerCallback(), timeToWait,
                           timer.TYPE_ONE_SHOT);

    // kick off the channel with our proxy object as the listener
    channel.asyncOpen(new saveAsListener(), null);
  },