Partly closes #496: add missing type

[hash0000]

Fix type missing. More info in issue #496.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

Thanks for opening a PR! Can you please add a unit test? We use tsd for our types.

[mcollina]

The type test is https://github.com/fastify/fastify-multipart/blob/master/types/index.test-d.ts

[hash0000]

Do the files() and file() support all options from the FastifyMultipartBaseOptions interface (except throwFileSizeLimit and limits, of course)?

[mcollina]

I think so, but verify with the code.

[hash0000]

I'm not sure, but did you mean this test?

[Uzlopak]

Lets run the tests. Your holistic approach to also test the same thing in javascript is apprecaited.

[mcollina]

lgtm

[mcollina]

lgtm