File uploads are never really deleted in ConfTool. You can always access them in the history of file uploads.
To restore the previous upload of a contribution which has been removed, deleted, or overwritten, please go to the list of submissions first:
Overview => Submissions & Reviews => List of Submissions
Open the details of the corresponding submission by clicking on its title. For the submitted files you will find a link to the “History of Uploads”. Click on this link and download the required file to your local storage medium.
Now "re-upload" this locally stored file for this contribution by clicking on upload in the list of submissions.