Unknown format in Input when creating X509Certificate2?

Question

I am attempting to create a X509CertificateCollection to use for validating my Nest client for ElasticSearch. I am attempting to create the first certificate like this:

private static readonly string Certificate = "mycert.cert";
private static X509Certificate2 cert = new X509Certificate2(Certificate);

However this is giving me the error

Interop.AppleCrypto.AppleCommonCryptoCryptographicException : Unknown format in import.

This is confusing me because having checked the documentation I thought I could create a X509Certificate using a file path. What am I doing wrong and how can I fix this issue?