Is there any specific requirements on server end to get WWW.uploadProgress to work?

Texture2D tex = (Texture2D)Resources.Load("back");  
byte[] bytes = tex.EncodeToPNG();
//Remove the unused texture
Destroy(tex);

//Create POST data
var form = new WWWForm();
form.AddField("authToken", "abc123");
form.AddBinaryData("levelImage", bytes);

WWW www = new WWW(API_ENDPOINT + "/levels", form);

while (!www.isDone)
{
    Debug.Log(www.uploadProgress.ToString("F4"));
    yield return null;
}

yield return www; //Execute the www request

if( www.error != null )
{
    Debug.Log(www.error);
}

Debug.Log("Image uploaded");

This code simply go from 0 to 1 but nothing between the values.

I have created a REST API in rails for running this (using default webrick server 1.3.1 with Ruby 1.9.2 at the moment).