[BUG] TypeScript Fetch client doesn't support multipart/form-data body

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Generated TypeScript code doesn't properly handle FormData, the definition below generates:

            if (file !== undefined) { 
                localVarFormParams.set('file', file as any);
            }

            localVarHeaderParameter['Content-Type'] = 'application/x-www-form-urlencoded';

When it should generate

const formData = new FormData();
formData.append("file", file);
localVarFormParams.body = formData;
localVarHeaderParameter['Content-Type'] = 'multiparm/form-data';

openapi-generator version

3.3.4

 "/api/Documents": {
      "post": {
        "tags": [
          "Documents"
        ],
        "operationId": "Upload",
        "consumes": [
          "multipart/form-data"
        ],
        "produces": [
          
        ],
        "parameters": [
          {
            "name": "file",
            "in": "formData",
            "required": false,
            "type": "file"
          }
        ],
        "responses": {
          "200": {
            "description": "Success"
          }
        }
      }

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Command line used for generation

docker run --rm -v ${PWD}:/local openapitools/openapi-generator-cli:v3.3.4 generate
-i "http://host.docker.internal/swagger/v1/swagger.json"
-g typescript-fetch
--type-mappings Date=string
-o /local/src/swagger

Steps to reproduce

Related issues/PRs

Suggest a fix

Start with the typescript-fetch generator and reproduce the issue using the supplied OpenAPI snippet and v3.3.4 Docker command. Compare the generated FormData handling and Content-Type with the expected multipart behavior; done means the generated client sends the file as multipart/form-data rather than application/x-www-form-urlencoded.