I think the document manager [1] covers this use-case rather well. Any app can register with the OS as being able to open a certain type of file. Then it's just a couple lines to show the user an "Open in..." prompt for a file. This is what Dropbox, Mail, and GDrive do and I think we're going to see a lot more of it in the future.

[1]: https://developer.apple.com/library/ios/documentation/FileMa...