The docs say:

dhparam: DH parameter file to use for DHE key agreement. Use openssl dhparam command to create it. If the file is invalid to load, it is silently discarded.

The fact that this seems to take a file instead of a buffer or string strikes me odd, as all other TLS options take these types. I've searched a bit and it seems there are no fs calls to read such a file. I see this in lib/_tls_common.js:

 if (options.dhparam) c.context.setDHParam(options.dhparam);

Could it be that the docs are just plainly wrong about this option?