http.ServeFile returning 404 on ECS

I have an endpoint where I need to serve the templates (csv files)

I have written the code for the same

func main() {
    router := InitRouter()
    log.Fatal(http.ListenAndServe(":8000", router))
}

func InitRouter() http.Handler {
    r := mux.NewRouter()

    r.HandleFunc("/downloadTemplate", DownloadTemplate).Methods(http.MethodGet)

    return r
}

func DownloadTemplate(res http.ResponseWriter, req *http.Request) {
    templateName := strings.TrimSpace(req.URL.Query().Get("templateName"))
    fileExtension := strings.TrimSpace(req.URL.Query().Get("fileExtension"))

    baseDirectoryPath, _ := os.Getwd()
    filePath := filepath.Join(baseDirectoryPath, "files", "templates")
    fileName := templateName + fileExtension

    res.Header().Set("Content-Disposition", mime.FormatMediaType("attachment", map[string]string{"filename": fileName}))
    res.Header().Set("Content-Type", "application/octet-stream")

    http.ServeFile(res, req, filepath.Join(filePath, fileName))
}

I am able to get the required file on my local machine
But when I hit the same endpoint on ECS, it returns 404 page not found
Response headers are:

{
    "Content-Disposition": "attachment; filename=filename.csv",
    "Content-Type": "text/plain; charset=utf-8",
    "Content-Length": 19
}

Here, Content-Disposition header has been set as it should be. But, the "Content-Type" header should have been "application/octet-stream" which is not

What am I missing here? Is there anything else which needs to be done to serve files from the golang server?