Cannot select private key when configuring EAP-TLS

Have just installed Ubuntu MATE 16.04 on a Raspberry Pi and am trying to configure EAP-TLS for use on the wired port.
~alexs/Downloads has a cert file (xxx.crt), a CA root file and a client key file ( bbbb.key)

When configuring EAP-TLS, I can select the client cert from my Downloads folder, the root CA associated with our RADIUS server from my Downloads folder but cannot even see the .key file containing the private key associated with the client cert.

The file selector option says DER,PEM, or PKCS#12 (*.der,*.pem,*.key
I've tried a .key version and a .p12 version, neither are visible.

When I use the finder to look at my downloads folder I can see all the files there owned by my userid / group with ug+rw and o+r permissions

Anyone else configured EAP-TLS? Clean install with latest kernel and all available patches installed
Rgds
Alex