Golang: multipart POST multipart request with io.Pipe - failing to get it right

Question

I am using GO multipart with io to upload a file to a local server and this function fails me every time. (This is just form request part)

func httpreq(uri string, params map[string]interface{}, fieldname, filename string, filebuf []byte) (*http.Request, error) {
    pr, pw := io.Pipe()
    writer := multipart.NewWriter(pw)
    go func() {
        defer pw.Close()
        r := bytes.NewReader(filebuf)

        part1, err := writer.CreateFormFile(fieldname, filename)
        if err != nil {
            return
        }
        if _, err = io.Copy(part1, r); err != nil {
            return
        }
        // encode params into Json
        var j []byte
        j, err = json.Marshal(params)
        if err != nil {
            return
        }
        part2, err := writer.CreateFormField("json")
        if err != nil {
            return
        }

        _, err = io.Copy(part2, bytes.NewBuffer(j))
        if err != nil {
            return
        }
        if err = writer.Close(); err != nil {
            return
        }
    }()
    req, err := http.NewRequest("POST", uri, pr)
    if err != nil {
        return nil, err
    }
    req.Header.Set("Content-Type", writer.FormDataContentType())
    fmt.Printf("---> %+v", req)
    return req, err
}

I am expecting request something like this

--f82711203f053f26bd2ea538ab381e30ec1468bff784e34174b11bb679dc
Content-Disposition: form-data; name="file"; filename="nov.txt"
Content-Type: application/octet-stream

   November 2019
Su Mo Tu We Th Fr Sa
                1  2
 3  4  5  6  7  8  9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30


--f82711203f053f26bd2ea538ab381e30ec1468bff784e34174b11bb679dc
Content-Disposition: form-data; name="json"

{"data_profile_id":"1","file_sha":"12345678","no_cache":true,"service_name":"test","tenant_id":"tenant"}
--f82711203f053f26bd2ea538ab381e30ec1468bff784e34174b11bb679dc--

but I get this

--c3b154dc079686ba162e0ab057c59580932ebab25294569201ef854394a7
Content-Disposition: form-data; name="file"; filename="102.pdf"
Content-Type: application/octet-stream


--c3b154dc079686ba162e0ab057c59580932ebab25294569201ef854394a7
Content-Disposition: form-data; name="json"

If I move out part

// encode params into Json
        var j []byte
        j, err = json.Marshal(params)
        if err != nil {
            return
        }
        part2, err := writer.CreateFormField("json")
        if err != nil {
            return
        }

        _, err = io.Copy(part2, bytes.NewBuffer(j))
        if err != nil {
            return
        }

then it seems to be giving me somewhat close to expectation

--f82711203f053f26bd2ea538ab381e30ec1468bff784e34174b11bb679dc
Content-Disposition: form-data; name="file"; filename="nov.txt"
Content-Type: application/octet-stream

   November 2019
Su Mo Tu We Th Fr Sa
                1  2
 3  4  5  6  7  8  9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30


--f82711203f053f26bd2ea538ab381e30ec1468bff784e34174b11bb679dc

Any experts here and what am I missing?