$.post('{YOUR_URL}', $(form).serialize(), function(r){
location.href = '{ROBOKASSA_URL}?order_id=' + r.order_id;
// или
$(robokassa_form).find('input[name="order_id"]').val( r.order_id );
$(robokassa_form).submit();
}, 'json');
If maxChunkSize is set to an integer value greater than 0, the File Upload plugin splits up files with a file size bigger than maxChunkSize into multiple blobs and submits each of these blobs to the upload url in sequential order.