Thunar

Thunar is a file manager, and with its plugin in Kupfer you can initiate a number of actions: empty trash, select file in thunar, show file properties, copy file to (...), move file to (...) and make symlink to file in (...).

In Thunar itself configure a custom action for selecting files in Kupfer: Create a new custom action called Send to Kupfer with the command kupfer %F and enable it for all files.

If you can, temporarily enable editable menu accelerators to assign a shortcut key to the new custom action. If you now select files in Thunar and press the shortcut key, Kupfer will be summoned with the files selected.