Need to import .cer file, attached in mail

[Plastilina_Ve]

Hello everyone!

I'm new around here, and this is my first doubt, I hope you can help me:

Someone sent me an email with a certificate file ".cer" attached to it. I can only see the file from my iPhone; I go to Gmail and there is a blank where the file should be, and when I download the email in the Mail app on OS X, the blank now says "Missing Plug-In".

So, I can clearly see it on the iPhone, but I tap on it, and nothing happens, nothing at all.

Thank you very much for your time, I hope there's a solution out there.

Happy weekend.