Add PositionType.FILE (gitlab4j#1117)

Author: 1cedsoda

src/main/java/org/gitlab4j/api/models/Position.java

@@ -12,7 +12,7 @@ public class Position implements Serializable {
 
     public static enum PositionType {
 
-        TEXT, IMAGE;
+        TEXT, IMAGE, FILE;
         private static JacksonJsonEnumHelper<PositionType> enumHelper = new JacksonJsonEnumHelper<>(PositionType.class,
                 false, false);