Microsoft Knowledge Base Article 299931 contains the following summary:
This step-by-step article explains how to use the new Microsoft FrontPage 2002 File Upload component while you write the form data to an Access database and send it to an e-mail address.
The File Upload component cannot be used with the Send To Database component in FrontPage 2002, because the former requires a page with an .htm file extension, and the latter requires .asp. Specifically, the File Upload component is processed on the server by the Shtml.dll file. When the user requests a page with an .asp file extension from the server, this indicates to the server that the page contains server-side ASP code and must be processed by the Asp.dll file. The server cannot process the page through both DLL files before sending the results to the client.