title: Redux Observable: How to return an action inside promise?
body: <p>I have an epic:</p>
  
  <pre><code>export default function uploadImage(action$, store) {
    return action$.ofType(userActions.UPLOAD_IMAGE)
      .mergeMap(action =&gt; {
        ...
        RNFetchBlob.fs.readFile(uploadUri, 'base64').then(data =&gt; {
          firebaseRef.putString(data).then(snapshot =&gt; {
            console.log('Uploaded a blob or file!');
            return Observable.of(userActions.updateImageURL(snapshot.downloadURL)); //&lt;----------
          })
        });
      })
  };
  </code></pre>
  
  <p>However, this throws <code>You provided 'undefined' where a stream was expected. You can provide an Observable, Promise, Array, or Iterable.</code> with reason.</p>
  
  <p>How would I wrap the callbacks inside an observable so that the epic doesn't break?</p>