WLAN EAP TLS which certificate file type

I try to create a wlan connection via eap tls with my own ca. I can select the device key (extension .key) but I cannot select my certificates. I tried pem crt and cer. What file type is needed and which extension can be found? SFOS 3.4.0.22

ok certificates must have the extension .crt or pem aaand: it worx