Feature Request: Add uploaded images to exported user files

[RayMacLewis]

When I create an export of my user files I will only get a zipped folder with my .md-files. But I was expecting to also get the uploaded images that are linked in the.md-files, at least these on the server itself.
Without checking the code it seems to me that there is no owner table used to protect the images from accessing them. In fact with the URL I can also see the pictures without even being logged in. See: #1601