Uploading file to AWS, UnityWebRequest.uploadProgress works in editor but not iOS

2018.4.13

Code (CSharp):

1.         byte[] bytes = System.IO.File.ReadAllBytes(path);
2.         string signedUrl = url;
3.         UnityWebRequest www = UnityWebRequest.Get(signedUrl);
4.         www.SetRequestHeader("Content-Type", "video/mp4");
5.      
6.  
7.         www.uploadHandler = (UploadHandler) new UploadHandlerRaw(bytes);
8.         www.method = "PUT";
9.  
10.         www.SendWebRequest();
11.         while (!www.isDone)
12.         {          
13.             callback(www.uploadProgress);
14.             yield return null;
15.         }

uploadProgress returns either 0 or 1 on iOS -- in Editor it behaves as expected during upload.

I've also tried using UnityWebRequest.Put() but same problem. Again, works in editor, not iOS -- haven't tried Android yet. I read it might be due to server not sending Content-Length in response headers, but GetResponseHeaders() fails until it's complete and Content-Length is there then. I tried to spoof Editor's user-agent but www.GetRequestHeader("User-Agent"); returns empty string.

Ideas? Uploading big files so progress bar is definitely needed.