Add sample code for file message content. (#319)

Author: tokuhirom

sample-spring-boot-kitchensink/src/main/java/com/example/bot/spring/KitchenSinkController.java

@@ -53,6 +53,7 @@
 import com.linecorp.bot.model.event.UnfollowEvent;
 import com.linecorp.bot.model.event.message.AudioMessageContent;
 import com.linecorp.bot.model.event.message.ContentProvider;
+import com.linecorp.bot.model.event.message.FileMessageContent;
 import com.linecorp.bot.model.event.message.ImageMessageContent;
 import com.linecorp.bot.model.event.message.LocationMessageContent;
 import com.linecorp.bot.model.event.message.StickerMessageContent;
@@ -187,6 +188,14 @@ public void handleVideoMessageEvent(MessageEvent<VideoMessageContent> event) thr
                 });
     }
 
+    @EventMapping
+    public void handleFileMessageEvent(MessageEvent<FileMessageContent> event) {
+        this.reply(event.getReplyToken(),
+                new TextMessage(String.format("Received '%s'(%d bytes)",
+                        event.getMessage().getFileName(),
+                        event.getMessage().getFileSize())));
+    }
+
     @EventMapping
     public void handleUnfollowEvent(UnfollowEvent event) {
         log.info("unfollowed this bot: {}", event);