Import of blog - not using /files but /wp-content/uploads

I've imported a bunch of wordpress/blogspot blogs to my WPMU setup.
But some blogs doesn't import the right way = When importing XML from a wordpress blog, the old media where located in wp-content/uploads, and therefore my WPMU setup is just showing a X instead of the picture.

This wasn't always the problem, but it is now. Why don't it change the file URL to /files as it should?