Support parsing multipart/form-data requests with different encoding

[mwannewitz]

It would be great if parsing of multipart/form-data requests with different content types could be supported.
https://swagger.io/docs/specification/v3_0/describing-request-body/multipart-requests/#specifying-content-type

requestBody:
  description: Contains a file and data attributes
  required: true
  content:
    multipart/form-data:
      schema:
        type: object
        required:
          - data
          - file
        properties:
          data:
            type: object
            required:
              - name
              - description
            properties:
              name:
                type: string
              description:
                type: string
          file:
            type: string
            format: binary
            description: Sample file
      encoding:
        file:
          contentType: text/csv
        data:
          contentType: application/json