You can use an app like ReaddleDocs for that, that's what I've always done on the iPhone. You can even download files from the web using the in-app browser.
As for posting pics on a forum you can upload the pics somewhere (Flickr, MobileMe, etc.) and post the link.
It's a poor solution. Downloading Pages gives you the ability to attach the file from Pages, but not from Mail. And when you click on a mailto link in Safari, it opens Mail, NOT Pages. Thus, you cannot send the attachment.
Now, if you're composing an email to a friend, would Readdle work? Sure. But tell me again why I need to go to an app other than Mail to actually, well, mail? It's decidedly annoying, unintuitive, and inconsistent with Apple's normally (highly) functional UI.