This works only if the user is running Netscape Navigator. It will not work with Microsoft Internet Explore 3.0. This is not a deficiency in Domino; the original MSIE 3.0 and 3.01 don't support web uploads. MSIE 3.02 has a file upload add-on that does support uploads.
Create an action hotspot (Create/Action/ActionHotspot) on the form with a formula of @Command([EditInsertFileAttachment]). No field needs to be added; a field will be automatically generated by Domino after the hotspot. Domino will create a Notes V2 style attachment (attachment shows up at bottom of page after a horizontal rule) when the user does this.