imprv: set content length in response header of attachment route

packages/app/src/server/routes/attachment.js

@@ -7,7 +7,6 @@ const logger = loggerFactory('growi:routes:attachment');
 
 const { serializePageSecurely } = require('../models/serializers/page-serializer');
 const { serializeRevisionSecurely } = require('../models/serializers/revision-serializer');
-
 const ApiResponse = require('../util/apiResponse');
 
 /**
@@ -229,6 +228,7 @@ module.exports = function(crowi, app) {
     res.set({
       ETag: `Attachment-${attachment._id}`,
       'Last-Modified': attachment.createdAt.toUTCString(),
+      'Content-Length': attachment.fileSize,
     });
 
     // download