body := &bytes.Buffer{}
    writer := multipart.NewWriter(body)
    part, err := writer.CreateFormFile(field, filename)
    if err != nil {
        return nil, err
    }
    _, _ = part.Write(file)
    _ = writer.Close()

    req, err := http.NewRequest("POST", serverUrl, body)
    if err != nil {
        return nil, err
    }
    req.Header.Set("Content-Type", writer.FormDataContentType())