Bug Description

The ajvFilePlugin type definition is incompatible with Fastify's ajv.plugins option, causing TypeScript errors when registering the plugin.

Current Type

export function ajvFilePlugin (ajv: any): void

Expected Type

export function ajvFilePlugin (ajv: Ajv): Ajv

Steps to Reproduce

import fastify from 'fastify'
import { fastifyMultipart, ajvFilePlugin } from '@fastify/multipart'

const app = fastify({
  ajv: {
    plugins: [ajvFilePlugin]  // TypeScript error here
  }
})

app.register(fastifyMultipart)

Error Message

error TS2769: No overload matches this call.
  The last overload gave the following error.
    Type '(ajv: any) => void' is not assignable to type 'Plugin<unknown> | [Plugin<unknown>, unknown]'.
      Type '(ajv: any) => void' is not assignable to type 'Plugin<unknown>'.
        Type 'void' is not assignable to type 'Ajv'.

Root Cause

Fastify's ajv.plugins option expects Plugin<unknown> type from @fastify/ajv-compiler, which is defined as:

export interface Plugin<Opts> {
    (ajv: Ajv, options?: Opts): Ajv;  // Must return Ajv
    [prop: string]: any;
}

The actual implementation in index.js correctly returns Ajv:

function ajvFilePlugin (ajv) {
  return ajv.addKeyword({...})  // addKeyword returns Ajv
}

But the type definition incorrectly declares the return type as void.

Proposed Fix

1. Add ajv to devDependencies for type definitions
2. Update types/index.d.ts:

import type Ajv from 'ajv'

// ...

export function ajvFilePlugin (ajv: Ajv): Ajv

Environment

• @fastify/multipart: 9.4.0
• fastify: 5.7.4
• TypeScript: 5.8.3