In iOS the same feature appears as "Files integration", which allows you to see an app's files from the "Files" application, and some applications can see all "Files Enabled/Allowed" applications in their file selection window.
Just checked with Dropbox, and yep, that's how it works. Files can access Dropbox transparently, and Dropbox can access any files which can be seen by the Files app.
If you allow that, the app works like the way the person you're replying to wants. If you deny that, the application works the way you want.
If one company have it, the other can implement it, too. There's no shame in copying a good feature, is it?