The selected file may not be a GPG key file or it might be corrupt.
The file looks valid in gedit. I also tried the file from the "testing" directory on the mirror.
I can't find the key file for these keys anywhere else, and trying to use anything like (with or without the "0x"):
apt-key adv --keyserver keyring.debian.org
doesn't work (gpg: no valid OpenPGP data found.).