If necessary, touch the arrow on the right of that menu to navigate to the “Add Attachment” option.
Pick that, and you’ll see your iCloud Drive files.
So you can use this, obviously, to attach anything you’ve stored within iCloud Drive; however, if you touch the “Locations” option in the upper-left, you can also enable your other available services, such as Dropbox or Box, by tapping “More.”
When you toggle on a particular service, it’ll then show up under that same “Locations” button. This means that you can quickly attach files from some third-party file-storage apps without having to navigate to the app in question to do so. Neato! With as much as I use Dropbox, this’ll be game-changing for me. Not because I actually do much work with it, you understand, but because those funny cat pictures aren’t going to share themselves.