Fixed issue 698: Set default attachment preview image in the code (and not only in layout) so that whne moving from a message to another we get the right preview

src/com/fsck/k9/activity/MessageView.java

@@ -1341,6 +1341,10 @@ public class MessageView extends K9Activity
             {
                 attachmentIcon.setImageBitmap(previewIcon);
             }
+            else
+            {
+                attachmentIcon.setImageResource(R.drawable.attached_image_placeholder);
+            }
 
             mHandler.addAttachment(view);
         }