webMethods Knowlegebase : RAD / openapi 3 - can't retrieve filename in multipart upload

Problem

We have a provider RAD for a file upload by a multipart/form-data POST.

The Open API3 specification is as follows :

(...)
paths: 
  '/docs':
    post: 
      tags: 
        - test
      summary: test upload multipart
      description: test upload multipart
      operationId: postTestMtp
      requestBody: 
        description: fichier
        content: 
          multipart/form-data: 
            schema: 
              $ref: '#/components/schemas/document'
        required: true
      responses: 
        '201':
          description: Created
        '400':
          $ref: '#/components/responses/400'
        '401':
          $ref: '#/components/responses/401'
        '403':
          $ref: '#/components/responses/403'
        '500':
          $ref: '#/components/responses/500'
components: 
  schemas: 
    document: 
      title: document
      type: object
      properties: 
        file: 
          type: string
          format: binary
(...)

The generated service has the following input (see attached image input.png) :

• requestBody (document)
  • _untitled_1 (document)
    • $part_1 (document)
      • contentType (string)
      • file (byte[])

The received content-type is OK, the contents of the file is OK too, the problem is that the filename sent by the client is lost, it is nowhere in the input pipeline.

The filename is sent by the client in a standard way :

------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="file"; filename="testmultipart.txt"
Content-Type: text/plain

(data)
------WebKitFormBoundary7MA4YWxkTrZu0gW--

How can we access the filename property of the “file” part ?