form FILE input?

Hello; I have been working on a project to work up a module in javascript for the sake of reading from input fields and converting them to a form with hidden fields. When the form with the hidden fields is submitted, the server will process the $_POST vars. But what if the page with the input elements available for the use to fill in has a file input field? The server has to see $_FILE input set for the upload to take place. So, the question is: How is the $_FILE variable established? Is it the enctype setting in the form itself or is it the presence of the file field explicitly? This is an issue because if I convert the file input to hidden fields will the server see $_FILE input? Perhaps this is beyond the scope of this list. If so, Thank you for attention JK If not Thank you for you time and attention JK