How to parse and extract MIME parts from a MIME message in Go?

To parse and extract MIME parts from a MIME message in Go, you can use the mime/multipart package from the Go standard library. Here's an example of how you can do it:

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"mime/multipart"
	"net/mail"
)

func main() {
	mimeMessage := `Content-Type: multipart/mixed; boundary="====Boundary===="

--====Boundary====
Content-Type: text/plain; charset="UTF-8"

This is the text part of the MIME message.

--====Boundary====
Content-Type: application/pdf
Content-Disposition: attachment; filename="document.pdf"

[PDF data here]

--====Boundary====--
`

	message, err := mail.ReadMessage(strings.NewReader(mimeMessage))
	if err != nil {
		log.Fatal(err)
	}

	mediaType, params, err := mime.ParseMediaType(message.Header.Get("Content-Type"))
	if err != nil {
		log.Fatal(err)
	}

	if strings.HasPrefix(mediaType, "multipart/") {
		mr := multipart.NewReader(message.Body, params["boundary"])

		for {
			part, err := mr.NextPart()
			if err != nil {
				log.Fatal(err)
			}
			defer part.Close()

			// Get the part's content type and disposition
			partMediaType, partParams, err := mime.ParseMediaType(part.Header.Get("Content-Type"))
			if err != nil {
				log.Fatal(err)
			}
			partDisposition, _, _ := mime.ParseMediaType(part.Header.Get("Content-Disposition"))

			// Handle different part types (e.g., text/plain, image/png)
			switch partMediaType {
			case "text/plain":
				body, err := ioutil.ReadAll(part)
				if err != nil {
					log.Fatal(err)
				}
				fmt.Println("Text Part:", string(body))
			case "application/pdf":
				// Read the attachment and perform necessary operations
				attachment, err := ioutil.ReadAll(part)
				if err != nil {
					log.Fatal(err)
				}
				fmt.Println("Attachment:", string(attachment))
			}

			// Handle the part disposition (e.g., attachment)
			if partDisposition == "attachment" {
				// Perform actions for attachments (e.g., saving)
			}
		}
	}
}

In this example, we define a MIME message as a string and use mail.ReadMessage to parse it into a mail.Message struct. We then use mime.ParseMediaType to get the media type and other parameters from the content type header.

If the media type is multipart, we create a multipart.Reader using multipart.NewReader and iterate over each part using NextPart. Within the loop, we can handle different part types accordingly (e.g., text/plain, application/pdf) by reading the part's content.

Remember to handle any errors that may occur during the parsing and extraction process.