Sharing attachments using Gmail sharing extension

Question

I'm attempting to share an image in my app using Gmail sharing extension. Everything works okay except for the recipient of the email who receives a malformed file.

Sample project can be downloaded from here.

Using iOS 8.3, Xcode 4.6.3, Gmail 4.0

Sharing code looks like this:

UIImage *image = [UIImage imageNamed:@"image"];
NSData *imageData = UIImagePNGRepresentation(image);

UIActivityViewController *activityVC = [[UIActivityViewController alloc] initWithActivityItems:@[imageData]
                                                                             applicationActivities:nil];

[self presentViewController:activityVC animated:YES completion:nil];

This presents the activity sheet like this:

Pressing Gmail button shows:

And the email arrives like this:

But, everything is okay if I try to use iOS's Mail.app: