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[eluser]Burak Guzel[/eluser]
You just need to remove the "<form>" from your view (the first line).

You are already calling form_open(), which correctly creates a POST form tag, instead of the default GET (which causes the error you are having).

Another unrelated problem: Your form_open() links to 'site/add_artifact', while in fact your controller is named 'site/add_artifacts'.