Parse PFFile download order iOS

I'm storing 5 PFFiles in an array and using getDataInBackgroundWithBlock to download those files from Parse.

The problem is the order at which they appear in the table view cells is different every time, presumably because the files are download at different speeds due to the different file sizes.

for (PFFile *imageFile in self.imageFiles) {
  [imageFile getDataInBackgroundWithBlock:^(NSData *imageData, NSError *error) {
    if (!error) {
      UIImage *avatar = [UIImage imageWithData:imageData];
      [self.avatars addObject:avatar];
      cell.userImageView.image = self.avatars[indexPath.row];
    }
  }];
}

The self.imageFiles array is in the correct order. How do I ensure that the images downloaded are added to the self.avatars array in the same order as the self.imageFiles?