Enable export feature for Dropbox paper documents

[TranceLove]

Checklist

• I've checked the feature request list and there's no request created for this purpose
• I've tried discussing this first in the discussion section to find out if there's something similar implemented (Or checked it by myself)

Is your feature request related to a problem? Please describe.
Dropbox Paper, that can be think as Dropbox's Google Documents, allows exporting files to designated format (export type(s) as defined by Dropbox). While we may never have the ability to act/open Dropbox Paper documents directly, we may instead provide a way for exporting these files to end-user/human readable formats.

Describe the solution you'd like
Enable export feature in plugin-dropbox instead of simply throwing Exception

Describe alternatives you've considered
N/A

Additional context
https://help.dropbox.com/organize/dropbox-paper-faqs#Is-Dropbox-Paper-free
https://www.dropbox.com/developers/documentation/http/documentation#files-export