Improve typescript types

[dwickern]

I've tried to correct the types by analyzing the source code and busboy's type definitions.

Fixes #251

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

[dwickern]

The value should be unknown, since it might be parsed as JSON based on the content-type provided by the caller. I've left it generic in case someone wants to assert its type.

[dwickern]

string | undefined because mime type get swallowed when we parse the value as JSON. I think this is a bug, but that's for another PR.

[mcollina]

lgtm

[leppaott]

Our working code wasn't valid after this:
await part.toBuffer();

Adding:

if ('file' in part) {
            imageBuffer = await part.toBuffer();

according to those unit tests this became valid but it wasn't at all obvious based on types.

Don't think the README example: https://github.com/fastify/fastify-multipart#handle-multiple-file-streams-and-fields

const parts = req.parts()
  for await (const part of parts) {
    if (part.file) {
      await pump(part.file, fs.createWriteStream(part.filename))
    } else {
      console.log(part)
    }
  }

will work anymore because part.file isn't available unless you do 'file' in part so IMO should revisit this.

[leppaott]

This should be modified on README as well. But this needs to be revisited to be on the type.

[climba03003]

Your problem is TypeScript issue.
TypeScript cannot restrict the type in certain format that's the cause.

The README example is certainly valid in Javascript.

[dwickern]

Yes, typescript wants you to use in for narrowing in this situation, which works for both JS and TS.
https://www.typescriptlang.org/docs/handbook/2/narrowing.html#the-in-operator-narrowing

A discriminator would perhaps be ideal:

export interface MultipartFile {
  type: 'file',
  ...
}
export interface MultipartValue<T = unknown> {
  type: 'value',
  ...
}

if (part.type === 'file') {
  part.toBuffer()
}