Photos Extra Share and .png

On Windows Phone 8 it is possible to register your app with the Share… extension point on the photo viewer. This allows your app to receive an image file and process it whether that is sending it to an online service or manipulating it on the device. I found recently an odd quirk with the feature. I setup the extension following the MSDN documentation. I wanted to test on my captured screenshots on the device so I went into the screenshots folder, selected an image and tapped share. While a range of system extensions were shown my custom app was not. I later discovered that it works in other albums. The reason as it turns out is that all screenshots are captured as .png files and camera images are .jpg and for whatever reason photo extras are not enabled for .png files. I searched online and there was a forum discussion but it is still awaiting a formal confirmation from Microsoft that this is expected behaviour.