Fileloader onProgress callback won't fire, what am I missing?

Hello,

I’m trying to load a 3D model from my server using three’s builtin ObjLoader. The onProgress callback is never called and I don’t understand why. Other than that, the model loads fine and everything works flawlessly.
The weird thing is the callback is called when I’m loading a local file. Could this be an issue with the way I deliver the file server-side?

Client looks like:
loader.load(url, onLoad, onProgress, onError);
with onProgress simply printing to the console.

Server looks like:
var readStream = fs.createReadStream(path);
stream.pipe(res);

Thanks everyone :).