Whoa. You've brought up a point that no one else has.
So far there doesn't seem to be any PDA-like tools on it to see onboard files (beyond pictures etc).
But if we go to a website with a file input button, then will iPhone Safari let us browse its file system for the upload? If so, how much of the file system will it let us see?
I remember Jerry Yang(new ceo of yahoo) during the Macwold keynote say they were going to be introducing their new Yahoo Go! services on iPhone. I have Yahoo Go! running on my razr v3xx, and it allows direct integration to Flickr. It's actually a really useful app, and it would be amazing on the iPhones huge screen. As for Apple allowing you direct integration to media on the phone through Safari remains to be seen.