Incorrect request body when 'multipart/mixed' is used

Q&A (please complete the following information)

• OS: Windows, macOS
• Environment: Nodejs 12, any browser
• Method of installation: npm
• Swagger-Client version: 3.18
• Swagger/OpenAPI version: OpenAPI 3.0

Content & configuration

Swagger/OpenAPI definition:

    requestBody:
        description: .....
        content:
          application/json:
            schema:
              $ref: 'certificate.yaml#/Certificate'
          multipart/mixed; boundary=BOUNDARY:
            schema:
              type: object
              properties:
                CertificateBody:
                  $ref: '#/Certificate'
                CerificateContent:
                  type: string
                  format: binary
          required: true

Swagger-Client usage:
Default configuration, just use a definition containing multipart/mixed

Describe the bug you're encountering

Multipart/mixed request is broken. When used, the request body is JSON and not multipart. It was working prior to version 3.10.6. All later versions including the latest have the same behavior.

To reproduce...

Steps to reproduce the behavior:

1. Compose a yaml using OpenAPI 3.0 and use multipart/mixed content type. Use two parts - one JSON, and the other string/binary.
2. Add the yaml in swagger url
3. Fill the JSON/file parts of the request and execute

Expected behavior

The request body should be multipart.

Current behavior

The request body is JSON and the request fails.

Screenshots

Additional context or thoughts

I think the issue is caused by this commit: https://github.com/swagger-api/swagger-js/commit/651e85713c90345508c3d32bfef63cba7fce7c5c
The condition for multipart/ is changed to 'multipart/form-data' which results in non-working multipart/mixed.

Start by reproducing the request with the OpenAPI 3.0 definition described in the issue, using one JSON part and one string/binary part. Compare multipart handling with the linked commit and verify that the generated request body is multipart/mixed rather than JSON; the issue is resolved when the described request executes successfully.