Since the release of Nexus 3.14.0, uploads of Apache Nifi Archives (NAR files) are rejected by Maven repositories when "strict content validation" is enabled. These files are generated by the nifi-nar-maven-plugin and are used for deploying features to Apache Nifi instances. (You can find lots of examples of these NAR files in Maven Central.)
In previous versions of Nexus this worked fine; when NAR files are uploaded to a 3.14.0 repository, however, they are detected as having content type application/vnd.iptc.g2.newsmessage+xml and are rejected.
I believe this is a result of an upgrade from Tika 1.14 to 1.19 as part of this commit; Tika added support for that content type as part of TIKA-2527 for a different file that also has the .nar extension.
This should be a relatively quick fix and I've created a pull request that should fix the issue.