[BUG][python-flask] binary response type is generated with rtype `file`

Description

When setting the response type as type: string and format: binary, the return type is generated as file, even though the documentation states that file is no longer a data type in OpenAPI 3.0, as mentioned here: https://swagger.io/docs/specification/data-models/data-types/#file

format: byte is correctly generated as bytearray.

This becomes a problem for us as we use custom templates to set the return type of functions ({{#returnType}} -> {{.}}{{/returnType}}), which will result in file, which is not valid in python, meaning the resulting generated code does not run.

openapi-generator version

Tested with 4.1.3, as well as these snapshots:
openapi-generator-cli-4.2.1-20191104.150105-22
openapi-generator-cli-5.0.0-20191024.154544-16

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Python binary response
  version: 1.0.0
paths:
  "/foo":
    post:
      responses:
        "200":
          description: "Success"
          content:
            application/octet-stream:
              schema:
                type: string
                format: binary

Command line used for generation

java -jar openapi-generator-cli-4.1.3.jar generate -g python-flask -i openapi.yml

Steps to reproduce

1. Copy yaml above to openapi.yml
2. Run java -jar openapi-generator-cli-4.1.3.jar generate -g python-flask -i openapi.yml
3. openapi_server/controllers/default_controller.py now contains a single function, with :rtype: file

Related issues/PRs

None which I could find

Suggest a fix/enhancement

I am unfamiliar with the code base, but what I could find was that here the type file is declared as a primitive of Python, which I don't think is the case. It also maps file to file. Presumably this should map to something like str or bytearray.

Start by reproducing the issue with the supplied OpenAPI YAML and python-flask generation command, then inspect PythonAbstractConnexionServerCodegen.java and the generated openapi_server/controllers/default_controller.py. Compare the binary response mapping with the correctly generated byte mapping. Done means the generated return type is valid Python rather than file, while the existing byte behavior remains unchanged.