To copy or move a file to a subdirectory of the current folder, in the Files tab on the right side of the window:
Select one or more files in the list.
Right-click a selected file and choose File actions > Copy/move file(s).
In the dialog, use the Action pulldown to indicate whether the file(s) should be copied or moved.
Select the subfolder in which to place the copied or moved file(s) from the To subfolder pulldown.
Press Copy/move.