How to send additional data using PLupload?

If you need to dynamically add parameters on every file upload you can do this:

uploader.bind('BeforeUpload', function(up, file) {
    up.settings.multipart_params = {

        "parameter1": "value1",
        "paremeter2": "value2"

    };
});

You can also use

uploader.settings.url = "upload.php?param1=whatever"

and just pass it as a get variable.

Have you tried using the setting for multipart_params? Add an additional option to your plupload.Uploader like so:

var uploader = new plupload.Uploader({
    runtimes : 'gears,html5,flash,silverlight,browserplus',
    browse_button : 'pickfiles',
    container : 'contact_container',
    max_file_size : '10mb',
    url : 'upload.php',
    flash_swf_url : '/plupload/js/plupload.flash.swf',
    silverlight_xap_url : '/plupload/js/plupload.silverlight.xap',
    filters : [
        {title : "Image files", extensions : "jpg,gif,png"},
        {title : "Zip files", extensions : "zip"}
    ],
    resize : {width : 320, height : 240, quality : 90},
    multipart_params : {
        "name1" : "value1",
        "name2" : "value2"
    }
});

You will then need to process the values in the file that handles the upload (upload.php by default). I think the values are captured by $_POST but you can use $_REQUEST just to be sure.

I've used jQuery to assign values on the fly, so instead of "name1" : "value1" you can use something like "name1" : $("#name1").val(), where #name1 might be an input elsewhere on the page.

Plupload's documentation is a little sparse for some of these settings.

You can use uploader.settings.multipart_params["name1"] = yourValue; but "name1" must be declared in uploader config :

multipart_params : {
    "name1" : "value1",
    "name2" : "value2"
}