As for now, if you drag'n'drop file to haruna window, it replaces the playlist content, but not added to it, and there is no way to change that behaviour anyhow. Only ways to fill playlist is to manually add files one by one with "Add" button (even it doesn't support selecting multiple files at once). Also, if you drag'n'drop multiple files to haruna window, it will only play one of them (the one it will count as first, I guess). // Just like it behaves when you specify multiple files on command line or, (what is actually the same), trying to open multiple files with it from dolphin