Server requirement for WWW.uploadProgress to work?

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).