fastify-multer | Fastify plugin for handling multipart/form-data | Runtime Evironment library

In your params, the "key" is in lowercase whereas while uploading your are using params.Key which should be params.key

Thanks to the creator of fastify-multer, Maksim Sinik, for helping me solve this silly issue. All credit goes to him. I found out that, in fact, fastify-multer works great with fastify-autoload and fastify-plugin. My issue was that in Postman, I wasn't allowing the auto-generated header content-length to be sent with the request:

Internally, fastify-multer uses npm module type-is to help discover the type of the request. If content-length is not sent, content-type is not sent as multipart/form-data and the type-is hasBody method returns null, which causes request.files to be undefined.

Moral of the story: if you want to test fastify-multer (or any other multipart data processor for that matter) with Postman, you must ensure the auto-generated content-length AND content-type header are enabled (selected):

I really hope this helps someone in the future. I was pulling my hair out! Thanks again to Maksim. You can find more of the work he's done on Github.