Howto do multipart/formdata upload in (new) generally available Logic App designer

Question

I have a Logic App in which I call an operation in API management to upload a document and several properties.

This works just fine in the 'old' Logic App designer.

When I switch to the new Logic App designer, the code is removed upon saving. So I need to rewrite the code in a format, so that the multipart/formdata upload keeps working. In the new designer a required property 'FormData' is shown in the interface, but which data should I enter here to get correct behaviour?

When I search online, I keep finding that formData should not be used in OpenApi v3.0.x and that I should use requestBody (which is exactly what I'm using in my current code). Can someone clarify?

API definition (OpenAPI JSON v3.0.1):

    "/documents/file": {
      "post": {
        "tags": [
          "Documents"
        ],
        "summary": "Create File Based Document",
        "description": "Create File Based Document",
        "operationId": "create-file-based-document",
        "parameters": [
          {
            "name": "Content-Type",
            "in": "header",
            "required": true,
            "schema": {
              "type": "string",
              "default": "multipart/form-data"
            }
          }
        ],
        "requestBody": {
          "content": {
            "multipart/form-data": {
              "schema": {
                "properties": {
                  "file": {
                    "type": "string"
                  },
                  "labelId": {
                    "type": "string"
                  },
                  "ownerId": {
                    "type": "string"
                  },
                  "description": {
                    "type": "string"
                  },
                  "filename": {
                    "type": "string"
                  }
                }
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "Document created",
            "content": {
              "application/json": {
                "example": {
                  "data": 0,
                  "meta": {
                    "status": "string",
                    "code": 0,
                    "requestDate": "string"
                  }
                }
              }
            }
          }
        }
      }
    },

Code in 'old' editor to call API:

{
    "inputs": {
        "method": "post",
        "body": {
            "$content-type": "multipart/form-data",
            "$multipart": [
                {
                    "body": "@body('Get_blob_content_(V2)')",
                    "headers": {
                        "Content-Disposition": "form-data; name=\"file\"; filename=@{body('Get_Blob_Metadata_(V2)')?['Name']}"
                    }
                },
                {
                    "body": "@variables('DocumentType')",
                    "headers": {
                        "Content-Disposition": "form-data; name=\"labelId\""
                    }
                },
                {
                    "body": "@body('get-databyusername')['data']['id']",
                    "headers": {
                        "Content-Disposition": "form-data; name=\"ownerId\""
                    }
                },
                {
                    "body": "@body('Parse_JSON')?['DocumentType']",
                    "headers": {
                        "Content-Disposition": "form-data; name=\"description\""
                    }
                },
                {
                    "body": "@body('Get_Blob_Metadata_(V2)')?['Name']",
                    "headers": {
                        "Content-Disposition": "form-data; name=\"filename\""
                    }
                }
            ]
        },
        "headers": {
            "Content-Type": "multipart/form-data"
        },
        "pathTemplate": {
            "template": "/api-rest/v1/documents/file",
            "parameters": {}
        },
        "api": {
            "id": "/subscriptions/<