Fix artifact extension retrieval

Do not rely on the packaging type to figure out what the extension of the
main artifact will be. So far, using `jar` and `war` packaging for `.jar`
and `.war` files worked by chance.

We know retrieve the actual extension as provided by Maven's
`ArtifactHandler`.

Fixes gh-2762

spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RepackageMojo.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -187,7 +187,7 @@ public class RepackageMojo extends AbstractDependencyFilterMojo {
 			classifier = "-" + classifier;
 		}
 		return new File(this.outputDirectory, this.finalName + classifier + "."
-				+ this.project.getPackaging());
+				+ this.project.getArtifact().getArtifactHandler().getExtension());
 	}
 
 	public static enum LayoutType {