feat: Preserve original format of images, change interpolation kernel

Closes #349

server/api/controllers/attachments/download-thumbnail.js

@@ -50,7 +50,7 @@ module.exports = {
       sails.config.custom.attachmentsPath,
       attachment.dirname,
       'thumbnails',
-      'cover-256.jpg',
+      `cover-256.${attachment.image.thumbnailsExtension}`,
     );
 
     if (!fs.existsSync(filePath)) {