add interface in uploader.js

src/server/service/file-uploader/uploader.js

@@ -29,6 +29,10 @@ class Uploader {
     return !!this.configManager.getConfig('crowi', 'app:fileUpload');
   }
 
+  deleteFiles() {
+    throw new Error('Implemnt this');
+  }
+
   /**
    * Check files size limits for all uploaders
    *