The three things you've mentioned all do different things.
Do you want to add the files (as something you can paste into a file manager to copy the files), or the file names (as text you can paste into a text editor) or the file contents (as text) to the clipboard?
You could create a button with the following MS-DOS command:
echo {filepath$} >> C:\Temp\FileList.txt
Select a file, click the button and the filename will be copied to a file called FileList.txt in the C:\Temp\ folder. Further clicks of the button will add to this text file.
This little program would do what you want. It monitors the clipboard and everything you copy is automatically added to it's window. You can then manipulate the list as desired.