Had a client with the same issue, but here extensions did not change; meaning i.e. the .doc did not change. However, when she tried to open the file she got the error.

She copied the files to a usb key and then copied one file back and renamed the file and then it opened. You might not need to copy the file back and forth, just try renaming the files and see if they open.

I haven't tested it myself but give it a whirl.