Howdy,
I am following the instructions at the section “Use different accounts on a single GitLab instance” found in https://docs.gitlab.com/ee/ssh/README.html#use-different-accounts-on-a-single-gitlab-instance, whilst I have no idea of which extension the config file must have. I am using Windows 10 and as I understand each file must have an extension. The docs only instruct me to save the file as ~/.ssh/config. What should I use as a file extension here?
Files do not necessarily need an extension, even on Windows: https://askubuntu.com/a/1188034
The file extensions generally help with niceties such as identifying the file type in file browsers, and the program to use, but you can use a simple text editor to save files with no extensions, and the data is still saved.
The instructions you are following appears to be written with Linux in mind. For discovering the equivalent filename and path that would work for OpenSSH clients on Windows, see https://superuser.com/a/1631971