In Tor browser, when you download a file you see a folder icon next to the download. In previous versions of Whonix, clicking this folder opened the file manager (Thunar.)
As of W18 workstation, clicking that folder does nothing.
Hypothesis is that the default file manager has not been updated to PCManFM-Qt.
A temporary workaround is to go to the pull-down menu, select the dispVM, and open its file manager (or a terminal) manually, then navigate to .tb/tor-browser/Browser/Downloads.