I remember writing this comment in my app a while ago:
// Adding the image to the attachment actually deletes the file (huh?!), so save it with a new name, and use that one instead of the original
I had an image in a folder that I attached to a notification. Then, when I tried to view the original image in the folder, it wasn't there. So, my fix was to make a copy of it (resized and quality lowered as it wasn't necessary to have a full-size image), and attach that file to the notification so my original image wasn't lost.
I think it's expected behaviour, but the Apple docs on attachments here don't mention it.