Turn FileType::get_from_client into an &self method.

Turn FileType::get_from_client into an &self method.

This enables the FileType object itself to store information about
where its data comes from, instead of _necessarily_ being constrained
to get it from the client.

Therefore, I've also renamed it, so that the method name doesn't imply
that the data always lives in the client.

This also means that every call to FileContents::update_items() must
pass through the FileType from its owning File, but that's no trouble.