How to extract MIME attachments from a received email message in Go?

To extract MIME attachments from a received email message in Go, you can use the net/mail and mime/multipart packages. Here's an example of how you can accomplish this:

package main

import (
	"io"
	"log"
	"net/mail"
	"mime/multipart"
	"os"
)

func main() {
	// Open the email file
	file, err := os.Open("email.eml") // Replace with the path to your email file
	if err != nil {
		log.Fatal(err)
	}
	defer file.Close()

	// Parse the email using net/mail
	msg, err := mail.ReadMessage(file)
	if err != nil {
		log.Fatal(err)
	}

	// Check if the email has multiple parts (which would indicate MIME attachments)
	if msg.Header.Get("Content-Type") == "multipart/mixed" {
		mr := multipart.NewReader(msg.Body, msg.Header.Get("Content-Type"))

		// Iterate over each part in the email
		for {
			p, err := mr.NextPart()
			if err == io.EOF {
				break
			}
			if err != nil {
				log.Fatal(err)
			}

			// Determine the filename of the part
			filename := p.FileName()
			if filename != "" {
				// Create a new file to save the attachment
				file, err := os.Create(filename)
				if err != nil {
					log.Fatal(err)
				}
				defer file.Close()

				// Copy the attachment data to the new file
				_, err = io.Copy(file, p)
				if err != nil {
					log.Fatal(err)
				}
				log.Println("Attachment saved:", filename)
			}
		}
	}
}

In this example, the email file is read using os.Open, and the mail.ReadMessage function from the net/mail package is used to parse the email. If the email has a Content-Type of "multipart/mixed", indicating the presence of attachments, a multipart.Reader is created using the email's body and Content-Type.

The code then iterates over each part in the email using mr.NextPart(), and checks if the part has a non-empty FileName() (indicating an attachment). A new file is created using os.Create, and the attachment data is copied to the new file using io.Copy. Finally, the filename of the attachment is printed to the console.

Make sure to replace "email.eml" with the path to your email file.