I've looked into dropbox a bit and can't seem to find a compelling reason to add support for this. Dropbox has their own app which allows you to interface with your dropbox, and you can easily send files from their app to Folder if you want to preview, email, make a copy, etc. It's not a technical reason, I am happy to do this if someone gives me a good reason to do so that dropbox won't already cover. So if that's you, drop me an email or comment here and let me know!
This goes for other feature requests as well. Thanks for using folder!