Flask codegen server does not correctly return files

Description

When trying to create an endpoint that returns a file, the generated server tries to serialize the file into JSON, after which it fails.

The error is TypeError: Object of type TextIOWrapper is not JSON serializable

Swagger-codegen version

I am using http://editor.swagger.io/#/ to generate the flask server

Swagger declaration file content or url

If you post the code inline, please wrap it with

swagger: '2.0'
basePath: /api/v1
schemes:
  - http
paths:
  /test/featureFile:
    post:
      tags:
        - cukegen
      summary: Generate feature file from Postman files
      description: Generate feature file from Postman files
      operationId: generateFeatureFile
      consumes:
        - multipart/form-data
      produces:
        - multipart/form-data
      parameters:
        - name: featureFileName
          in: formData
          description: Name of Feature file to generate
          required: false
          type: string
        - name: postmanCollectionFile
          in: formData
          description: Postman Collection file to upload
          required: true
          type: file
        - name: postmanEnvironmentFile
          in: formData
          description: Postman Environment file to upload
          required: true
          type: file
      responses:
        '200':
          description: Succesful operation
          schema:
            type: file
        '400':
          description: Invalid Parameter(s)
          schema:
            $ref: '#/definitions/ErrorResponse'
        '500':
          description: Internal Server Error
          schema:
            $ref: '#/definitions/ErrorResponse'
definitions:
  ErrorResponse:
    type: object
    properties:
      errorMessage:
        type: string

Steps to reproduce

Try to return a file from the cukegen controller endpoint and call the API.

Start with the generated Flask server and the cukegen controller endpoint described in the issue, using the supplied Swagger declaration to reproduce the request. Confirm the response path for the file-returning endpoint and verify that calling it returns the file without the TextIOWrapper JSON serialization error.