Bug 13799 JAR exporter exports class files from output folder
There was a problem with the output folder not being detected as such.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/jarpackager/JarFileExportOperation.java
List files= new ArrayList(resources.length);
		for (int i= 0; i < resources.length; i++)
			if (resources[i].getType() == IResource.FILE && isClassFile(resources[i]))
				files.add(resources[i]);
		return files.iterator();
	}
	/**
	 * Answers whether the given resource is a Java file.
	 * The resource must be a file whose file name ends with ".java".
	 * 
	 * @return a <code>true<code> if the given resource is a Java file
	 */
	boolean isJavaFile(IResource file) {
		return file != null
			&& file.getType() == IFile.FILE
			&& file.getFileExtension() != null
			&& file.getFileExtension().equalsIgnoreCase("java"); 
	}
	/**
	 * Answers whether the given resource is a class file.
	 * The resource must be a file whose file name ends with ".class".
	 * 
	 * @return a <code>true<code> if the given resource is a class file
	 */
	boolean isClassFile(IResource file) {
		return file != null
			&& file.getType() == IFile.FILE
			&& file.getFileExtension() != null
			&& file.getFileExtension().equalsIgnoreCase("class"); 
	}