This allows you to directly share files in your Dropbox, with your contact, without leaving the app.
How to send files in your Dropbox within Facebook Messenger
Tap on the three dot “More” button to view a list of apps which support Facebook Messenger. Now tap on Dropbox to download and install it from the Google Play Store, and sign in . You will see that the chat conversation has a Dropbox button. Tap on the button to navigate your Dropbox folders and view files without leaving messenger. This option allows you to send any file (images, videos, docs) you have in the cloud to your contact directly.
The feature is already working with the latest version of Messenger.