HTTP Header form fields and file upload

I'm trying to automate the process of uploading files from a client to
a webserver. I know that this can't be done by setting the value of
the file field in an html form.

So far, the plan is to mimic the upload form by sending an http
request to the server. I can do this without the file upload no
problem, but I can't figure out how to include the file as well. Can
anyone please help?

Chris