Upload a file with POST request golang - go

Question

I'm new to golang and I'm trying to write a function that uploads a file with a post request to telegram for a bot I'm writing.
I've tried with this code but the error I'm getting from telegram is
Bad Request: there is no photo in the request.
I've searched on the net for how to do that, but none of what I found helped me through the problem.
func SendPostRequest (url string, filename string) []byte {
file, err := os.Open(filename)
if err != nil {
log.Fatal(err)
}
defer file.Close()
response, err := http.Post(url, "binary/octet-stream", file)
if err != nil {
log.Fatal(err)
}
defer response.Body.Close()
content, err := ioutil.ReadAll(response.Body)
if err != nil {
log.Fatal(err)
}
return content
}
The function I'm calling the SendPostRequest from is
func (e Engine) SendPhoto (filename string, chatId int64) APIResponse {
var url = fmt.Sprintf("%ssendPhoto?chat_id=%d", e.baseUrl, chatId)
var content []byte = SendPostRequest(url, filename)
var response APIResponse
json.Unmarshal(content, &response)
return response
}
EDIT:
The link to the Telegram bot api I'm using in the code is https://core.telegram.org/bots/api
And the api method is https://core.telegram.org/bots/api#sendphoto

Answer 1

After some digging I figured it out with this
import (
"bytes"
"io"
"mime/multipart"
"net/http"
"path/filepath"
)
// content is a struct which contains a file's name, its type and its data.
type content struct {
fname string
ftype string
fdata []byte
}
func sendPostRequest(url string, files ...content) ([]byte, error) {
var (
buf = new(bytes.Buffer)
w = multipart.NewWriter(buf)
)
for _, f := range files {
part, err := w.CreateFormFile(f.ftype, filepath.Base(f.fname))
if err != nil {
return []byte{}, err
}
_, err = part.Write(f.fdata)
if err != nil {
return []byte{}, err
}
}
err := w.Close()
if err != nil {
return []byte{}, err
}
req, err := http.NewRequest("POST", url, buf)
if err != nil {
return []byte{}, err
}
req.Header.Add("Content-Type", w.FormDataContentType())
client := &http.Client{}
res, err := client.Do(req)
if err != nil {
return []byte{}, err
}
defer res.Body.Close()
cnt, err := io.ReadAll(res.Body)
if err != nil {
return []byte{}, err
}
return cnt, nil
}