APIC-EM Certificate - Not valid extension .crt – Cisco DevNet Developer Support

It says that for PEM certs a .crt is acceptable but getting a red box when trying to drag and drop it is saying "Not a valid extension". Having clients CA group to generate a PEM based .crt certificate as they do with other appliances but its not being accepted by APIC-EM.
Any knowledge around why it would not like something that it seems to be documented . Are there checks in the background that may be obscure that it is failing and its a general error against the cert and not so much the extension specifically...

1.3 doc snippet:

Step 7

if you choose PEM, then perform the following tasks:

For the Certificate field, import the PEM file by dragging and dropping this file into the Drag n' Drop a File Here field.

Note For a PEM file, it must have a valid PEM format extension (.pem, .cert, .crt). The maximum file size for the certificate is 10KB

APIC-EM Certificate - Not valid extension .crt