Files that have already been uploaded can be copied individually or together:
Click onto one file or select the files you want to copy. You can do this by pressing and holding down the left mouse button and move it over the files that you want to copy. You can release the mouse button after you finished selecting the files.
With a right click a window opens and you select "copy". Now open the destination folder. Click the right mouse again in the folder and choose "insert" in the window.