In Flex how can I set the <input type=file/> value

Question

I have an AIR app that download a webpage using HTMLLoader and I want to set the value of a form input element with a type=file. I've read that this is not allowed for security reasons but I've also found a Firefox extension that does it. Below is the js function they use. If a Firefox extension can do it, is there now way for an AIR app to do it? Thanks

Ad.prototype.generate_Image = function(image_path) {
    var $mb = jQuery.noConflict();
    var imgFrame = $mb("iframe[name='UIAdCreativeImage_Frame']", window.content.document)[0].contentDocument;
    var fileBox = $mb(".UIAdCreativeImage_FileUpload", imgFrame)[0];
    fileBox.value = image_path;
    imgFrame.forms[0].submit();
}