MIME4J-270 Using "alternative" as default subtype.
diff --git a/dom/src/main/java/org/apache/james/mime4j/message/MultipartBuilder.java b/dom/src/main/java/org/apache/james/mime4j/message/MultipartBuilder.java
index e33f979..8546ab5 100644
--- a/dom/src/main/java/org/apache/james/mime4j/message/MultipartBuilder.java
+++ b/dom/src/main/java/org/apache/james/mime4j/message/MultipartBuilder.java
@@ -66,6 +66,7 @@
     private MultipartBuilder() {
         this.bodyParts = new LinkedList<Entity>();
         this.parameters = new LinkedList<NameValuePair>();
+        this.subType = "alternative"; // the default value; see getSubType() JavaDoc
     }
 
     public MultipartBuilder use(final BodyFactory bodyFactory) {
diff --git a/dom/src/test/java/org/apache/james/mime4j/message/MultipartBuilderTest.java b/dom/src/test/java/org/apache/james/mime4j/message/MultipartBuilderTest.java
index 3579fb8..8193465 100644
--- a/dom/src/test/java/org/apache/james/mime4j/message/MultipartBuilderTest.java
+++ b/dom/src/test/java/org/apache/james/mime4j/message/MultipartBuilderTest.java
@@ -39,7 +39,17 @@
 public class MultipartBuilderTest {
 
     @Test
-    public void testBasis() throws Exception {
+    public void testDefaults() {
+        MultipartBuilder builder = MultipartBuilder.create();
+
+        Assert.assertEquals("alternative", builder.getSubType());
+        Assert.assertNull(builder.getPreamble());
+        Assert.assertNull(builder.getEpilogue());
+        Assert.assertEquals(0, builder.getCount());
+    }
+
+    @Test
+    public void testBasis() {
         MultipartBuilder builder = MultipartBuilder.create("stuff");
         builder.setPreamble("Hello");
         builder.setEpilogue("Bye");