Multipart. Got a null response for non-nullable field

I get an error when sending a multipart request with a file. But If I send a request bypassing the gateway, there is no error.

Response

{
  "errors": [
    {
      "message": "map[string]interface {} is not an Upload",
      "path": [
        "uploadFile",
        "input",
        "file"
      ],
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "extensions": {
        "selectionPath": [
          "uploadFile"
        ],
        "selectionSet": "{ uploadFile(input: $input) { id url } }",
        "serviceName": "...",
        "serviceUrl": "http://..."
      }
    },
    {
      "message": "got a null response for non-nullable field \"uploadFile\"",
      "path": [
        "uploadFile"
      ]
    }
  ],
  "data": null
}

Request payload

-----------------------------386949902811154848193528235860
Content-Disposition: form-data; name="operations"

{"query":"mutation UploadFile($input: UploadFileInput!) {\n  uploadFile(input: $input) {\n    id\n    url\n  }\n}","variables":{"input":{"file":null,"entity":"user-avatar"}}}
-----------------------------386949902811154848193528235860
Content-Disposition: form-data; name="map"

{"1":["variables.input.file"]}
-----------------------------386949902811154848193528235860
Content-Disposition: form-data; name="1"; filename="cropped-1712928276777.jpeg"
Content-Type: image/jpeg

...

Schema

scalar Upload

input UploadFileInput {
    entity: String!
    file: Upload!
}

type File {
    id: String!
    url: String!
}

type Mutation {
    uploadFile(input: UploadFileInput): File! @authorized
}

{
  "data": {
    "service": {
      "name": "...",
      "schema": "directive @authorized on FIELD_DEFINITION\ninput UploadFileInput {\n\tentity: String!\n\tfile: Upload!\n}\n\ntype File {\n\tid: String!\n\turl: String!\n}\n\ntype Mutation {\n\tuploadFile(input: UploadFileInput): File! @authorized\n}\ntype Service {\n\tname: String!\n\tversion: String!\n\tschema: String!\n}\n\ntype Query {\n\tservice: Service!\n}\nscalar Upload\n"
    }
  }
}

It looks like it's a gateway's side issue or I'm doing something wrong. I will be glad to hear your opinions or get advice on how to solve this. Thanks!

movio / bramble

Multipart. Got a null response for non-nullable field #262