How to Use multipart in Go

multipart in Golang is a powerful tool for creating multipart HTTP requests. This can be particularly useful when sending data that includes both text and file content.

Solution

To create a multipart form request, follow these steps:

1. Instantiate a bytes.Buffer and a multipart.Writer object.
2. Use the multipart.Writer to create multipart.Part objects, one for each part of the request.
3. Set the Content-Type header of the request to the value returned by writer.FormDataContentType().
4. Write the contents of the request parts to the multipart.Writer.
5. Call writer.Close() to complete the request.

Example

In your example, you would create a multipart mixed request as follows:

body := &bytes.Buffer{}
writer := multipart.NewWriter(body)

part, err := writer.CreatePart(textproto.MIMEHeader{"Content-Type": {"application/json"}})
if err != nil {
    // handle error
}

part.Write(jsonStr)

writer.Close()

req, err := http.NewRequest("POST", "blabla", body)
if err != nil {
    // handle error
}

req.Header.Set("Content-Type", "multipart/mixed; boundary=" writer.Boundary())

Bonus Tip: Using cURL

You can also generate a multipart request using cURL with the following command:

curl -F "field=value" -H "Content-Type: multipart/mixed; boundary=boundary" http://1.1.1.1/blabla