Inconsistent Multipart type when some fields are not files

🐛 Bug Report

When sending multiple fields in the form-data, Typescript types them all as files, even if it’s not the case

To Reproduce

Steps to reproduce the behavior:

server.post<{Body: {field1: Multipart, field2:Multipart}}>(...)

http -f :3000/mulitpart field1=foo [e-mail]

With this call, we submit a file and a string. But due to the Multipart type, they’re all typed the same and we can’t access the .value property containing the string (as in this test).

Expected behavior

We could take advantage of TypeScript conditional typing to allow user to properly type its body.

interface MultipartFile {
    filename: string;
}
interface MultipartString {
    value: string;
}
type Multipart<T = true> = T extends string ? MultipartString : MultipartFile;

const mult = {} as Multipart<string>
mult.value // OK
mult.filename // error

const multFile = {} as Multipart
multFile.value // error
multFile.filename // OK

Playground to see it in action

Your Environment

• node version: 14
• TypeScript version: 4.0.2
• fastify version: 3.3.0
• fastiify-multipart version: 3.2.0
• os: Linux

[StarpTech]

Definitly. We could create a section called “Typescript usage” and link it to https://github.com/fastify/fastify-multipart/blob/master/test/index.test-d.ts

[mage1k99]

Still there is no section for typescript usage 🤔