[BUG][typescript-fetch] Complex values are serialized as the string [object Object] in multipart/form-data

Created by: verokarhu

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

Complex types are incorrectly serialized as the string [object Object] when they are used as a part in a multipart/form-data request body.

openapi-generator version

4.3.1 / 5.0.0-beta2 / 2ec96f7f

OpenAPI declaration file content or url

---
openapi: "3.0.0"
info:
  title: Bug test case
  version: "1.0"
paths:
  /example:
    post:
      operationId: exampleOperation
      requestBody:
        content:
          multipart/form-data:
            schema:
              properties:
                complexvalue:
                  properties:
                    value1:
                      type: number
                    value2:
                      type: string
                  type: object
              type: object
      responses:
        "201":
          description: Created

Generation Details

java -jar openapi-generator-cli.jar generate -g typescript-fetch -i testcase.yml -o generated

Steps to reproduce

Related issues/PRs

Suggest a fix

The generated model (models/ExampleComplexvalue.ts, 21-34) is correct:

export interface ExampleComplexvalue {
    /**
     * 
     * @type {number}
     * @memberof ExampleComplexvalue
     */
    value1?: number;
    /**
     * 
     * @type {string}
     * @memberof ExampleComplexvalue
     */
    value2?: string;
}

However, the generated serialization code (apis/DefaultApi.ts, 53-55) is incorrect:

        if (requestParameters.complexvalue !== undefined) {
            formParams.append('complexvalue', requestParameters.complexvalue as any);
        }

As you can see from the documentation of FormData.append(), all other types than Blob or subclasses will be converted into string:

value
The field's value. This can be a USVString or Blob (including subclasses such as File). If none of these are specified the value is converted to a string.

The correct way to serialize this would be to convert the complex type into a JSON Blob before serializing it:

new Blob([JSON.stringify(requestParameters.complexvalue)], { type: "application/json", }

This should not be done to simple string types though, since they need to be appended as-is and should have the default text/plain Content-Type.