You may need to turn off strict content type validation for the repository. The error message suggests content type detection is choosing the incorrect type.
I’m having the same issue and we turned off strict content type validation for the repo but we are still getting the error. We are running version 3.73.
I don’t have a nar file, but searching suggests its simply a zip. So I rename an archive and use the automatically created maven-releases repository which has strict content-type validation disabled:
curl -v -X PUT -T foo.nar http://localhost:8081/repository/maven-releases/g/a/1/a-1.nar
If I turn on Validate that all content uploaded to this repository is of a MIME type appropriate for the repository format in the repository configuration and retry then the result is as you describe: