Strange behaviour with request.file() when filename in form is not specified

### 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

4.23.2

### Plugin version

8.1.0

### Node.js version

20.x

### Operating system

macOS

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

Sonoma 14.2

### Description

I encountered a strange behavior in the `request.file()` method when I send a multipart (using `form-auto-content`) without specifying the `filename` of the sent file. The method returns `undefined`. I do not know if it is a `form-auto-content` issue, if this is the expected behaviour or not. If it is expected I'll close this.

### Steps to Reproduce

Reproduction example: https://codesandbox.io/p/devbox/fastify-multipart-filename-nxnqgd

- Setup a fastify server with the `@fastify/multipart` plugin with a route calling `req.file()`
- Setup a simple client using `form-auto-content` and appending a file specifying the `contentType` but not the `filename`
- Call the endpoint

### Expected Behavior

I expect an error (if multipart with part missing the filename are considered malformed) or otherwise the parsed file.