Sharing a .jpg base64 file on Instagram results in a black story

Hello :)

From `RNStoryShareModule.java` :
```
private String generateFileName(){
    Random r = new Random();
    int hash = r.nextInt(999999);

    return "image-" + hash + ".png";
  }
```

It seems, we only generate `.png` files even if we are trying to share a base64 .jpg file.
It results in a black screen on instagram story.

`react-native-story-share: v0.6.0`