How to Upload a File with a POST Request in Go?

Upload a File with a POST Request in Go

Uploading a file via a POST request is a common task when developing web applications. This becomes even more important when working with Telegram bots, which require files to be uploaded as part of API calls.

Unfortunately, using a simple http.Post function may result in errors like "Bad Request: there is no photo in the request." To resolve this issue, you need to send the file using the multipart/form-data content type. Here's how you can achieve that:

1. Create a Content Structure:

   Define a content struct to represent the file's metadata and data:

   <code class="go">type content struct {
       fname string
       ftype string
       fdata []byte
   }</code>

2. Multipart Form Builder:

   Use multipart.NewWriter to create a new multipart form builder:

   <code class="go">var buf = new(bytes.Buffer)
   var w multipart.NewWriter(buf)</code>

3. Add File Data:

   Iterate over the files you need to upload and create a form part for each one:

   <code class="go">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
       }
   }</code>

4. Close Form:

   Once you have added all files, close the multipart form builder:

   <code class="go">err := w.Close()
   if err != nil {
       return []byte{}, err
   }</code>

5. Create HTTP Request:

   Create a new HTTP request using http.NewRequest:

   <code class="go">req, err := http.NewRequest("POST", url, buf)
   if err != nil {
       return []byte{}, err
   }</code>

6. Set Content Type:

   Set the Content-Type header to indicate that you are sending multipart/form-data:

   <code class="go">req.Header.Add("Content-Type", w.FormDataContentType())</code>

7. Send Request:

   Send the HTTP request using an HTTP client:

   <code class="go">client := &http.Client{}
   res, err := client.Do(req)
   if err != nil {
       return []byte{}, err
   }</code>

8. Read Response:

   Read the response body:

   <code class="go">cnt, err := io.ReadAll(res.Body)
   if err != nil {
       return []byte{}, err
   }</code>

By following these steps, you can successfully upload files with POST requests in Go, including when working with Telegram bots.

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