[Go] Unable to specify Content-Type for form data file upload

Description

For a Swagger spec of

      "post": {
        "summary": "upload",
        "description": "Upload a file.",
        "operationId": "uploadUsingPOST",
        "consumes": [
          "multipart/form-data"
        ],
        "produces": [
          "application/json"
        ],
        "parameters": [
          {
            "name": "file",
            "in": "formData",
            "description": "file",
            "required": true,
            "type": "file"
          },

the generated code will always set the Content-Type of the file to be application/octet-stream.

Some services expect the Content-Type of the upload file to be set correctly. Therefore, I would like to be able to set the Content-Type of a file upload.

This seems to be a result of the CreateFormFile() function from the stdin MIME library.
https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/go/client.mustache#L210

Swagger-codegen version

v2.4.10

Command line used for generation

docker run -v $(pwd)/output:/output --rm swaggerapi/swagger-codegen-cli generate \
  -i https://my.internal.service.com/api-docs \
  -l go \
  -o /output

Suggest a fix/enhancement

Start with modules/swagger-codegen/src/main/resources/go/client.mustache around line 210 and inspect how CreateFormFile assigns the upload MIME type. Trace the generated Go client's multipart form-data path and determine how a caller could specify the file Content-Type. Done means generated clients can send the requested file MIME type instead of always using application/octet-stream, with coverage for the upload case.