Select a file in the file manager. Open Kupfer and search for the object Selected File, which represents the file or files that are selected.
Instead of searching, you can press Ctrl+G to directly focus the selected file. See Command Keys and Accelerator Keys for more information.
The selected file plugin works with the Nautilus file browser and requires that the extension kupfer_provider.py is installed (it is normally installed together with Kupfer). After installing it for the first time, Nautilus must start anew before it is active.