I have a custom Wordpress plugin that handles many things including download pages for my products. The links are not the path to the actual PHP page to my plugin. For example...
Create a new shortcode (maybe empty that does nothing) and put it on some page you created from WordPress. Then, form action will be the URL to new page where you send data you need to send and method will be post. You can handle the data you sent in new shortcode function in your plugin using $_POST['data']
This might help you as an alternative solution.