fix(types): add optional value to MultipartFile for onFile

[imharjot]

The README documents setting part.value inside an onFile handler when using
attachFieldsToBody: 'keyValues', but MultipartFile has no value property, so that
example fails to typecheck with TS2339 (Property 'value' does not exist on type 'MultipartFile').

Adds an optional value?: unknown to MultipartFile. unknown (not any) keeps it
type-safe and matches the existing Record<string, unknown> usage in this file; it's
optional because a file part only gets a value when one is assigned in onFile. Kept
MultipartFile non-generic to avoid rippling generics through Multipart/MultipartFields.

The existing type test asserted req.body.file had no value, so that assertion is flipped
and the onFile test handler now assigns part.value to cover the regression.

Fixes #574