How to set a custom option of type “file” when add

Using my own controller, I'm adding a product to the Magento cart. It has 3 custom options: 2 dropdown options (color and size) and a file option (design). The code adding the product to the cart is

// obtain the shopping cart
$cart = Mage::getSingleton('checkout/cart');

// load the product
$product = Mage::getModel('catalog/product')
    ->load($productId);

// do some magic to obtain the select ids for color and size ($selectedSize and $selectedColor)
// ...

// define the buy request params
$params = array(
    'product'       => $productId,
    'qty'           => $quantity,
    'options'       => array(
        $customOptionSize->getId()  => $selectedSize,
        $customOptionColor->getId() => $selectedColor,

        // set the file option, but how? 
    ),
);

// add this configuration to cart
$cart->addProduct($product, $paramObject);
$cart->save();

// set the cart as updated
Mage::getSingleton('checkout/session')->setCartWasUpdated(true);

My question is: How do I attach a certain file to the design option?

The file has already been transferred to the server side (actually via the request). I could, however, fake uploading if this would be required. But until now I have not found a single source of information on setting file custom options...

My best guess from a tour through the Magento sources, is that the buy request needs some additional data (not in the options, but in the params object), like: option_123_file => something, but what exactly is needed I did not figure out yet. This part of the Magento sources is rather, uhh, not so straight forward. Thanks for any help.

Ok finally figured this out. The params array needs special entry to tell the custom option with the key "options_xx_file_action" what to do with a file ('save_new' or 'save_old'). This would look like:

$params = array(
    'product'       => $productId,
    'qty'           => $quantity,
    'options'       => array(
        $customOptionSize->getId()  => $selectedSize,
        $customOptionColor->getId() => $selectedColor,
    ),
    'options_'.$customOptionDesign->getId().'_file_action'=>'save_new',
);

Obviously, you will need to add the file to the post request (via form or therelike). The name of the file should be "options_xx_file". For example, in my case my $_FILES looked like:

Array (
[options_108_file] => Array
    (
        [name] => i-like.png
        [type] => application/octet-stream
        [tmp_name] => C:\xampp\tmp\phpAAB8.tmp
        [error] => 0
        [size] => 6369
    )

)