Convert .cer to .crt file

I had a certificate in the .cer format, which I wanted to install as a trusted certificate authority on my Linux Mint computer. I found some articles online, which showed, how to install a personal CA certificate, but these guides always used .crt files. I followed these articles with my .cer file, but success was not be achieved. So I had to convert the .cer file to a .crt file, which I did with the following command:

openssl x509 -inform PEM -in Fortinet_CA_SSL.cer -out Fortinet_CA_SSL.crt