The image attached is just "Attachment-1", no extension. How do I specify one ?

```
    NSData *compressedImage = UIImageJPEGRepresentation(self.resultImage, 0.8 );

    UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:@[ @"Check this out!", compressedImage ] applicationActivities:nil];

    [self.navigationController presentViewController:activityViewController animated:YES completion:nil];

```