create an attachment.js file in an apiv3 folder

src/server/routes/apiv3/attachment.js

@@ -0,0 +1,37 @@
+const loggerFactory = require('@alias/logger');
+
+const logger = loggerFactory('growi:routes:apiv3:attachment'); // eslint-disable-line no-unused-vars
+
+const express = require('express');
+
+const router = express.Router();
+
+/**
+ * @swagger
+ *
+ *     /attachment:
+ *      get:
+ *        tags: [Attachment]
+ *
+ */
+
+module.exports = (crowi) => {
+  const accessTokenParser = require('../../middlewares/access-token-parser')(crowi);
+  const loginRequired = require('../../middlewares/login-required')(crowi);
+  const Attachment = crowi.model('Attachment');
+
+  router.get('/attachment', accessTokenParser, loginRequired, async(req, res) => {
+
+    try {
+      const id = req.params.id;
+      const attachment = await Attachment.findById(id);
+      return res.apiv3({ attachment });
+    }
+    catch (err) {
+      logger.error('attachment not found', err);
+      return res.apiv3Err(err, 404); // 400?
+    }
+  });
+
+  return router;
+};