Web build: WWW downloads correctly, but www.assetBundle is Null

I have a bundle of about 10 .bytes files.

I follow the Unity WWW example.

I get www.isDone=true, and www.bytesDownloaded is the correct size, so it downloads something.

www.assetBundle is always null.

I use Dropbox Public folder as Server.

I get no error messages.

  void Start()
     {
         StartCoroutine(LoadBundle());
     }
     
     IEnumerator LoadBundle()
     {
         string downloadPath = FilePath.Url() + FilePath.Dir +
             DataLib.AssetBundlesDir + FilePath.Dir + "mainbundle";
     
         using (WWW www = new WWW(downloadPath))
         {
             yield return www;
             if (www.error != null)
                 throw new Exception("WWW download had an error:" + www.error);
             AssetBundle bundle = www.assetBundle;
     
             Debug.Log("wwwResult: is done: " + www.isDone.ToString() +
                 ", progress: " + www.progress.ToString() + ", bytesDownloaded: " + www.bytesDownloaded.ToString());
     
              // www.isDone is true
             //www.bytesDownloaded are correct
             //but www.assetBundle is Null
                 
         }
     }