Golang sending binary file over TCP via HTTP/REST

I'm writing a manager for cloud support of legacy IOT devices in Go, the devices establish a tcp connection with the manager, then that connection gets held open for bi-directional communication. The api for the devices is REST(ish) and the commands are written over the TCP pipe. For any HTTP/JSON commands that send a json payload, I have everything working, but yesterday I started working on firmware update, which requires I send a binary file instead of json.

I've been loving golang but this is a bit of a stretch for me. I've tried:

1. Use Postman POST, works.
2. Tried to use the same approach as the commands that have JSON payloads, but no luck.
3. Try to simplify and mimic what Postman is doing in Go to get to working Go code. I took out the error checking for brevity, but no errors are thrown:

func downloadFirmware() {   file, _ := os.Open(`c:\users\wayne\development\go\accesscontrol\core\firmware\myfile.img`)   body := &bytes.Buffer{}   writer := multipart.NewWriter(body)   part, _:= writer.CreateFormFile("file", "firmware.img")   io.Copy(part, file)   _ = writer.Close()   request, _:= http.NewRequest("POST", `http://192.168.85.249/eidc/download`, body)   request.SetBasicAuth(config.Credentials.UserName, config.Credentials.Password)   request.Header.Set("Content-Type", writer.FormDataContentType())   request.Header.Set(`Host`, config.OutboundConfig.PrimaryHostAddress)   request.Header.Set(`User-Agent`, `Listener`)   request.Header.Set(`Accept-Encoding`, `gzip, deflate, br`)   request.Header.Set(`Connection`, `keep-alive`)   request.Header.Set("Content-Length", fmt.Sprint(body.Len()))   client := &http.Client{}   response, err := client.Do(request)   byteBody, _ := io.ReadAll(response.Body)   fmt.Println(string(byteBody))   response.Body.Close()   file.Close()}

Monitoring with Wireshark I see the file get pushed, but the response from the device is failure, error downloading file.

I keep wondering if there's some termination character I need to add to the file or some other way of indicating the end of the file? I've checked the content length that gets generated and it matches Postman.