Schema validation breaks when attachFieldsToBody is set to 'keyValues'

[smaspe]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the regression has not already been reported

Last working version

7.7.3

Stopped working in version

8.0.0

Node.js version

18

Operating system

macOS

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

13.6

💥 Regression Report

When setting {attachFieldsToBody: 'keyValues'}, the content of the file is attached to the body, using the name of the field.

According to the documentation here: https://github.com/fastify/fastify-multipart#json-schema-body-validation, we'd expect a string.

However, since version 8.0.0 (specifically, this particular change here: c195d5f#diff-e727e4bdf3657fd1d798edcd6b099d6e092f8573cba266154583a746bba0f346L176, the object is attached as a Buffer.

Using JSON Schema, as far as I know, it is not possible to represent a Buffer object.

Steps to Reproduce

• Install version 8.0.0
• Try to define a body schema that validates a buffer?

Here is a short example of a server:

import fastifyMultipart from "@fastify/multipart";
import fastify from "fastify";

const app = fastify({ ignoreTrailingSlash: true });
app.register(fastifyMultipart, {
  attachFieldsToBody: "keyValues",
});

app.post(
  "/test",
  {
    schema: {
      body: {
        type: "object",
        properties: {
          myFile: { type: "string" },
        },
      },
    },
  },
  async (request, reply) => {
    reply.send();
  }
);

app.listen({ port: 3000 });

It works fine in 7.7.3. With 8.0.0, myFile is now a Buffer, and this server returns this:

{
    "statusCode": 400,
    "error": "Bad Request",
    "message": "body/myFile must be string"
}

Expected Behavior

• body.myFile should be a string, or there is a documented clear way of defining a Buffer in the JSON Schema validation

[gurgunday]

Good spot, the docs shouldn’t say that

[gurgunday]

You can use the object type for buffers