Maven Puts Arbitrary Extension Definition in JAR Manifest by Default.

I'm using the latest Maven release. When I build my project, the resulting Jar file's manifest contains an Extension-Name attribute along with Specification and Implementation attributes. The POM contains no mention that this project is a Java optional package – an "extension" (or an extension of any other kind).
I don't know why Maven is doing that.
If Maven is doing this by default for some reason, it absolutely shouldn't. Maven should not identify my Jar as an optional package unless I explicitly say so. Jars are only extensions if explicitly created as such.
The name it uses for the extension name is the POM's <artifactId>. That's not even a UID!