How do I associate file types with an iPhone application?

On the subject of associating your iPhone app with file types.

In this informative question I learned that apps could be associated with custom URL protocols.

That was almost one year ago and since then Apple introduced 'Document Support' which goes a step further and allows apps to associate with file types. There is a lot of talk in the documentation about how to set up your app to launch other appropriate apps when it encounters an unknown file type. This means the association doesn't work out of the box for any app, like the URL protocol registering did.

This leads me to the question: have system apps like Safari or Mail implemented this system for choosing associated applications, or will they do nothing, as before?

Source: Tips4all