UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36

Steps to reproduce the problem:
I am making a packaged app for use by journalists - currently this is being waited for by people to test from Washington, Paris and Netherlands (we already have other products installed on site)
1. i have a blob 
2. i convert to an arraybuffer
3. i send blob from a frame to main window (in order to save using the file system API available in main window) with

fileReader.onload = function() {
            var arrayBuffer = this.result;
            var data = {data:"test", command:"saveblob", name:name,
                mimetype:"audio/wav", ab:arrayBuffer};
            appWindow.postMessage(data, appOrigin, [data.ab]);
        };
        fileReader.readAsArrayBuffer(blob);
4. N.B. i also tried
appWindow.postMessage(data, appOrigin, [arrayBuffer]);

What is the expected behavior?
i expected the event.data to contain data

What went wrong?
app main window does receive the message but event.data is null

WebStore page: n.a.

Did this work before? N/A 

Chrome version: 52.0.2743.116  Channel: stable
OS Version: OS X 10.11.2
Flash Version: Shockwave Flash 22.0 r0

i couldn't find where to post this request - hope this is the correct place!