Apply suggestions from code review

Co-authored-by: delvh <dev.lh@web.de>

src/main/java/envoy/data/Attachment.java

@@ -35,7 +35,14 @@ public class Attachment implements Serializable {
 		 * 
 		 * @since Envoy Common v0.1-beta
 		 */
-		VOICE
+		VOICE,
+
+		/**
+		 * This attachment type denotes a document.
+		 *
+		 * @since Envoy Common v0.1-beta
+		 */
+		DOCUMENT
 	}
 
 	private final byte[]			data;