Use the correct API for the filepicker channel

www/filepicker/main.js

@@ -41,10 +41,34 @@ define([
             var Utils = config.modules.Utils;
 
             nThen(function (waitFor) {
-                config.modules.SFrameChannel.create($('#sbox-filePicker-iframe')[0].contentWindow,
-                    waitFor(function (sfc) {
+                // The inner iframe tries to get some data from us every ms (cache, store...).
+                // It will send a "READY" message and wait for our answer with the correct txid.
+                // First, we have to answer to this message, otherwise we're going to block
+                // sframe-boot.js. Then we can start the channel.
+                var msgEv = Utils.Util.mkEvent();
+                var iframe = $('#sbox-filePicker-iframe')[0].contentWindow;
+                var postMsg = function (data) {
+                    iframe.postMessage(data, '*');
+                };
+                var whenReady = waitFor(function (msg) {
+                    if (msg.source !== iframe) { return; }
+                    var data = JSON.parse(msg.data);
+                    if (!data.txid) { return; }
+                    // Remove the listener once we've received the READY message
+                    window.removeEventListener('message', whenReady);
+                    // Answer with the requested data
+                    postMsg(JSON.stringify({ txid: data.txid, language: Cryptpad.getLanguage() }));
+
+                    // Then start the channel
+                    window.addEventListener('message', function (msg) {
+                        if (msg.source !== iframe) { return; }
+                        msgEv.fire(msg);
+                    });
+                    config.modules.SFrameChannel.create(msgEv, postMsg, waitFor(function (sfc) {
                         sframeChan = sfc;
                     }));
+                });
+                window.addEventListener('message', whenReady);
             }).nThen(function () {
                 var updateMeta = function () {
                     //console.log('EV_METADATA_UPDATE');