[BUG] Multipart/form-data encoding object requires contentType

Description

Validation fails when multipart request description contains encoding without contentType.

Using openapi-generator-cli@1.0.12-4.3.0

OpenAPI declaration file content or url

       "requestBody": {
          "content": {
            "multipart/form-data": {
              "schema": {
                "type": "object",
                "properties": {
                  "file": {
                    "type": "string",
                    "format": "binary",
                    "nullable": true
                  }
                }
              },
              "encoding": {
                "file": {
                  "style": "form"
                }
              }
            }
          }
        },

Command line used for generation

openapi-generator generate -i swagger.json -g typescript-redux-query -o src/openapi-client

Steps to reproduce

• Swashbuckle.AspNetCore generates encoding object with style: "form" when using IFormFile to accept file uploads.

[HttpPost("{id}/files")]
public Task<ActionResult<EntityWithNameDto>> PostFiles(int id, IFormFile file, CancellationToken ct)

• While specifying content type for multipart requests is allowed, it is not required: https://swagger.io/specification/#encodingObject
• Validation fails with the following error: "...requestBody.content.encoding.contentType is missing"

Suggest a fix

After removing the encoding element entirely, validation passes. It should be possible to specify encoding without content type in this context.

Start with the openapi-generator generate -i swagger.json -g typescript-redux-query entry point and reproduce the validation failure using the multipart/form-data declaration shown in the issue. Trace the validation of the request body's encoding object; done means an encoding containing style: "form" without contentType validates successfully.