How to sanitize an $_FILES array

You can use the WordPress native function for that: wp_handle_upload.

My plugin has its own directory structure for saving media items, and i could not find how to make wp_handle_upload to store it in a by me specified location.

And, how to sanitize the $_REQUEST var that holds the value of a multiple selection box?

So, maybe that’s the problem. You have to use the standard way.

Can you explain me how to define a non-standard path in that case?