However, to fully integrate dropbox into pantheon-files, you must download a small plugin. In the terminal:
sudo apt-get install pantheon-files-plugin-dropbox
You may need to logout or restart at this point. (Different people have experienced different accessibility at this point.)Next, open the preferences in files. Navigate to the Extensions menu and select the Dropbox box. You may need to logout or restart again at this point. If you've done everything correctly, you should be able to secondary-click (right-click) a file in pantheon-files, and select from the Dropbox options.