Facebook's Developer Conference is scheduled to start today so this is probably one of many bits of Messenger and Facebook news you'll read in the next 48 hours. Dropbox just announced that it now integrates in Messenger to allow you to share any media files or documents from your cloud to friends and group chats.
If you already have the Dropbox app installed on your phone, you'll see it as a source for sharing under Messenger's "More" menu just like in the first screenshot above. You can then browse your Dropbox files, pick the one to share, and send it. If it's an image, gif, or video, it will be immediately embedded and previewed inside the conversation. Other file types show an "open" button which takes the recipient either to the Dropbox app (if it's installed on their phones) or mobile site and gives them the option to preview and save.
If you made it through the post despite those stock iPhone images at the top, we apologize deeply. Dropbox didn't provide any Android screenshots of the feature in action. It should work as pictured on Android too, and you'll find the links to both required apps below.