Automate Upload of a file to a website

I am writing a VB.NET application to automate the submission of real estate listings to a website.  I have done most of the project using IE automation and mshtml object.  I have discovered a problem that I now realize is quite common.  I need to upload a file using the form on the website. The input type="file" does not allow one to set the value attribute (for the filename) for security reasons.  I now realize that I need to create a HTTPRequest object from scratch and attach the file to that but I can't seem to find a solid example of how to do so.  For example, how can I tell what the contents of the request should look like in the first place? Also this is a password protected site.  Please help!