[JavaSpring] @MultiParam generated for multipart/form-data part with complex data instead of @MultiPart

Description

When the OpenAPI 3.0 YAML definition contains a multipart/form-data with a part referencing a complex schema, then the generated server-side interface defines a @MultiParam for the given part instead of a @MultiPart.

Swagger-codegen version

3.0.16

Swagger declaration file content or url

openapi: 3.0.0
info:
  version: "0.1"
paths:
  /testbug/:
    post:
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                firstPart:
                  $ref: "#/components/schemas/MyInput"
                secondPart:
                  type: string
                  format: binary
      responses:
        "200":
          description: OK
components:
  schemas:
    MyInput:
      type: object
      properties:
        myInputField:
          type: string

Command line used for generation

java -jar swagger-codegen-cli-3.0.16.jar generate -l spring -o output -i swagger.yaml

Steps to reproduce

Run above command on above OpenAPI 3.0 YAML content and check generated file output/src/main/java/io/swagger/api/TestbugApi.java

Related issues/PRs

None.

Suggest a fix/enhancement

Root cause seems bug in resource handlebars/JavaSpring/formParams.mustache in swagger-codegen-generators-1.0.16.jar.

Run the reported swagger-codegen CLI command with the supplied OpenAPI YAML and inspect output/src/main/java/io/swagger/api/TestbugApi.java. Then read resource handlebars/JavaSpring/formParams.mustache to trace how the complex multipart part is rendered. Done means the generated interface uses @MultiPart for firstPart instead of @MultiParam.