ExtJS file upload to NodeJS using CORS

I have a form with a filefield I want to upload to a NodeJS + Express server. I managed sending the file to my server using form.submit but the thing is I can't get a successful response from it...the message I get looks like this

{success:false,message:"Blocked a frame with origin "http://localhost:1841" from accessing a cross-origin frame."}

I managed avoiding this message and getting a successful return in some other places using JSONP requests, but I can't figure out how to solve this when trying to upload a file. Is there any way to upload a file using JSONP?

I tried enabling CORS in my NodeJS + Express backend but with no success (I used express-cors and cors modules), I get the same failure message.

This is what my submit function looks like

me.up('form').submit({
    url:'http://localhost:3000/parseExcel',
    waitMsg:'Leyendo archivo...',
    success:function(form, resp){
        console.log(resp);
        var data = JSON.parse(resp);
        console.log(data);
    },
    failure:function(form, o){
        console.log(form)
        console.log(o)
    }
})

And this is the backend function that is called on the form submit

router.post('/parseExcel', function(req, res){
    var obj = xlsx.parse(req.files.fileupload.path);
    var data = obj[0].data;
    var result = {
        success:true,
        data:data
    }

    var callback = req.query.callback;
    if (callback) {
        res.setHeader('Content-Type', 'text/javascript');
        res.send(callback + '(' + JSON.stringify(result) + ')');
    } else {
        res.setHeader('Content-Type', 'application/json');
        res.send(JSON.stringify(result));
    };
})