Issue on validation when exposing a multipart/form-data API

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## The feauture or bug you are proposing
Issue on validating a `multipart/form-data` request

## The description of the bug or the rationale of your proposal
The schema definition of a `mutlipart/form-data` part requires the following body structure:
```
{
additionalProperties: false,
properties: {
part: {
description: 'Binary content of the file to upload',
format: 'binary',
type: 'string',
},
},
required: ['part'],
type: 'object',
}
```
Calling the endpoint with a binary file uploaded through the `part` field does return a validation error which states: 'body.part must be string`

## The expected result for your bug
I expect that this validation error should not happen

### Your environment

node: 8.15.0

custom-plugin-lib: 4.3.2

os: 20.04.5 LTS (Focal Fossa), Ubuntu

Reproduce the multipart/form-data endpoint using the schema shown in the issue, uploading binary content through the part field. Trace the validation path for that request and verify the fix by confirming it no longer reports that body.part must be a string.

Written by the indexing model from the issue text.