Facebook Messenger users will be able to find Dropbox inside Messenger's extensions menu. From there, you'll be able to send photos, videos, and other files to whoever you're chatting with. If you send a video, an image, or a GIF, Messenger will be able to preview it inline; for any other type of file, you'll have to jump out to Dropbox itself.
Dropbox is definitely one of the smarter integrations we've seen so far in Messenger. Sending files to another person is something people already do with frequency, so being able to do that in a mostly-native way — without having to jump into one app, get a link, make sure the link works, and then send it to the other person — is a real convenience.
Expect to see a lot more news around Messenger tomorrow. That's when Facebook's annual F8 developer conference kicks off, and messaging is believed to be a big focus. While chatbots will likely be the highlight of the event, it wouldn't be surprising if new integrations are announced, too.