[BUG] aspnetcore generation multipart/form-data error

Bug Report Checklist

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

Description

When generating aspnetcore skeletton for json interface containing an element of "multipart/form-data:" with object type, the result controller file contains an unexpected method signature with UNKNOWN_BASE_TYPE as parameter Type and parameter name.

openapi-generator version

openapi-generator .3.3.4
openapi-generator .4.01

OpenAPI declaration file content or url

"post": {
"requestBody": {   
"content": {
    "multipart/form-data:": {
        "schema": {
            "type" : "object",
            "properties": {
                "memo": {
                   "type": "string",
                 },
                 "fileToUpload": {
                     "type": "string",
                     "format": "binary"
                  }
              }
          }
      }
}
}
}

The one which works:

"post": {
"requestBody": {   
content": {
      "multipart/form-data:": {
        "schema": {
          "type": "string",
          "format": "binary"
        }
      }
}
}
"


##### Command line used for generation

java -jar openapi-generator-cli.jar generate -i  file1.openapi.json --generator-name  aspnetcore -c file1.aspnetcore.config.json -o tmp/

##### Steps to reproduce

launch the generation, the generated controller file will have "UNKNOWN_BASE_TYPE"
as variable type and name.

##### Related issues/PRs
no

##### Suggest a fix

Start with the aspnetcore generation entry point and the generated controller file produced by the supplied OpenAPI declaration and command. Reproduce the multipart/form-data object case, compare it with the single binary-string case, and consider the issue done when the controller no longer contains UNKNOWN_BASE_TYPE as the parameter type or name.