Support multiple contentTypes in encoding multipart/form-data

### Question

Hi !

I have tried to upload an image that can be either a png or a jpeg, but It didn't work for me.

Here's a demo of a requestBody that I used to achieve the following according to [OpenAPI encoding specification](https://spec.openapis.org/oas/v3.1.0#encoding-object):
```
requestBody:
description: The avatar file
required: true
content:
multipart/form-data:
schema:
type: object
properties:
file:
type: string
encoding:
file:
contentType: image/png, image/jpeg
```

Unfortunatly, I'm getting this error:

`Validation warning: Inconsistency encountered when parsing ``: 'image/png, image/jpeg' could not be parsed as a Content Type. Content Types should have the format '/'. [context: codingPath=.paths['/profiles/{id}/avatar'].post.requestBody.content['multipart/form-data'].encoding.file.contentType, contextString=, subjectName=]warning: Validation warning: Inconsistency encountered when parsing ``: 'image/png, image/jpeg' could not be parsed as a Content Type. Content Types should have the format '/'. [context: codingPath=.paths['/profiles/{id}/avatar'].post.requestBody.content['multipart/form-data'].encoding.file.contentType, contextString=, subjectName=]Error: Invalid content type string: 'image/png, image/jpeg', must have 2 components separated by a slash.`

Can you tell me what I did wrong ?

Thanks !

Start by reproducing the supplied OpenAPI document and trace validation of encoding.file.contentType from the reported parsing error. Check the OpenAPI encoding specification for the expected representation, then add or update focused validation coverage so multiple media types are accepted without warnings or errors.

Written by the indexing model from the issue text.