This one single line differs in this PR with master changes:
https://github.com/sonatype/nexus-oss/pull/313/files#diff-8d61410e84836f8b521ba48172e01ca6R177

And this is due to having Tika@master vs MimeUtil2@nexus-2.7.x. In short, Tika is better doing it's job, as it recognizes even XHTML as `application/xhtml+xml`, and hence, content validation rejects it (as Nexus expects XML in file having extension "xml", not HTML or it's subtype), while MimeUtil2 recognizes XHTML as XML, and sees this as "correct" content.