M2Mqtt client side certification

I am using the m2mqtt .NET library, and I am currently having an issue validating the client side cert. I understand that this is not currently available with this library, how can I validate the server side. The website says that I should enter a valid cert using the Resources dir. .der/.crt/.cer are not valid resource types... how can add them as valid certs (I know that I don't need them all, I have just tries to add them).

via Chebli Mohamed