If you intend to use TLS, folder should contain two files named key.pem and cert.pem:
key.pem
contains the private key.cert.pem
contains the certificate. These can be created using OpenSSL with the following set of commands:
openssl genrsa -out key.pem
openssl req -new -key key.pem -out csr.pem
openssl x509 -req -days 9999 -in csr.pem -signkey key.pem -out cert.pem
rm csr.pem
The server checks if they exist at startup and uses the according protocol (HTTP/HTTPS).