Make ImageDataSource use PNG

This makes the code compatible with OpenJDK 8 (while currently it will
only work on Oracle JDK 8). PNG is probably the better choice anyway
because it is lossless.

Author: veithen

.travis.yml

@@ -1,6 +1,5 @@
 dist: trusty
 language: java
-# There are test cases that use ImageIO with JPEG and that only work on Oracle JDK.
 jdk:
   - oraclejdk8
 install: true

axis-rt-core/src/main/java/org/apache/axis/attachments/ImageDataSource.java

@@ -37,7 +37,7 @@ public class ImageDataSource implements DataSource {
     protected static Log log =
         LogFactory.getLog(ImageDataSource.class.getName());
 
-    public static final String CONTENT_TYPE = "image/jpeg";
+    public static final String CONTENT_TYPE = "image/png";
 
     private final String name;
     private final String contentType;