MK4

Was working on a piece for quite some time... as everyone does, I saved my piece as I was finishing up. A PDNSAVE file was created on my desktop as always, however whenever I try to open this on Paint.net it fails to open and displays the message, 'this image type is not recognised and cannot be opened'. I have got no idea how to fix this -- has anyone got any ideas? (and yes, my paint.net is also updated to the latest version)