Can you even find the file with its encrypted name in your backup? The file would be in the cache folder.
If you do find it, download the file from common space into a workspace, then load the file from a normal (non-PTC-product controlled) folder into Pro/E and overwrite the workspace file of the same name.
Unless your designers throw away a lot of their work, checking in every day or at least weekly should not increase the data vault size significantly. Even if it does, you can remove unreferenced files to clean out the stale files.