Type value does not exist on MultipartFile when using onFile

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the bug has not already been reported

### Fastify version

5.2.1

### Plugin version

9.0.3

### Node.js version

23.7.0

### Operating system

Windows

### Operating system version (i.e. 20.04, 11.3, 10)

11

### Description

When using `onFile`, the part does not have typings for `value`

So this code from the readme is not valid. As such I am providing this github repo as the reproducible example:

```js
async function onFile(part) {
const buff = await part.toBuffer()
const decoded = Buffer.from(buff.toString(), 'base64').toString()
part.value = decoded // set `part.value` to specify the request body value
}

fastify.register(require('@fastify/multipart'), { attachFieldsToBody: 'keyValues', onFile })

fastify.post('/upload/files', async function (req, reply) {
const uploadValue = req.body.upload // access file as base64 string
const fooValue = req.body.foo // other fields
})
```

### Link to code that reproduces the bug

https://github.com/fastify/fastify-multipart

### Expected Behavior

Typings for `value` on `MultipartFile`