Hi,
what do you mean with "Preview"?
* Sending of a proof?
* Or previewing the personalized message content locally in the client?
Where are the files stored?
The user running the MTA would need to have access to the file.
i.e. if the file is in a local directory on the same machine where Campaign Client console is installed, MTA won't be able to see it.
If it's on a network share etc., then that would have to be mounted to the correct location on the server