Use GM_xmlhttpRequest to POST data on Chrome?

I'm writing a user script to take an image from a page, and upload it to a server. The script works fine in FF (Greasemonkey and Scriptish), but when I use Chrome (using Tampermonkey or Ninjakit), it does not send the data, it sends the string * [object Object] * instead.

Here is my script:

// ==UserScript==
// @id             myid
// @name           myname
// @version        1.0
// @namespace      ohadcn
// @author         Ohad Cohen
// @description    mydescription
// @include        https://*
// @grant          GM_xmlhttpRequest
// @require        https://code.jquery.com/jquery-2.0.3.min.js
// @run-at         document-end
// ==/UserScript==

function getBase64Image(img) {
    var canvas = document.createElement("canvas");
    canvas.width = img.width;
    canvas.height = img.height;
    var ctx = canvas.getContext("2d");
    ctx.drawImage(img, 0, 0);
    var dataURL = canvas.toDataURL("image/png");
    return dataURL.replace(/^data:image\/(png|jpg);base64,/, "");
}

img=$("img[alt=myImage]").get(0);

img.onload=function(){
    var img64=getBase64Image(img)
    var _data=new FormData();
    _data.append("image64",img64);

    GM_xmlhttpRequest({
    method: "POST",
    url: "http://myserver.org/mysscript.py",
    headers: {
    "Content-Type": "multipart/form-data"
    },
    data:_data,
    onload: function(response) {
             console.log ("gut response");
         $("#input").get()[0].value=response.responseText;
    }
    });
}

Both Tampermonkey and Ninjakit do send the request. In Tampermonkey, I get a response, in Ninjakit I don't (onload is never called).

But they do not send the actual image encoded with base64 - when I read the data - the server gets [object Object] as the POST body (Instead of data body, I can't get devtools network panel to show requests made by GM_xmlhttpRequest, so I checked it on the server side).

It might be that FormData and multipart/form-data are not well supported on those platforms. Need to look into it more (later).

Meanwhile, try the more typical approach; use application/x-www-form-urlencoded or JSON.

EG:

GM_xmlhttpRequest ( {
    method:     "POST",
    url:        "http://myserver.org/mysscript.py",
    data:       "image64=" + encodeURIComponent (img64),
    headers:    {
        "Content-Type": "application/x-www-form-urlencoded"
    },
    onload:     function (response) {
        console.log ("gut response");
        $("#input").get()[0].value=response.responseText;
    }
} );