How to handle multipart MIME messages (e.g., email attachments) in Go?

To handle multipart MIME messages, including email attachments, in Go, you can use the mime/multipart package provided in the Go standard library. The package provides utilities to parse and manipulate multipart MIME messages.

Here's an example of how to handle multipart MIME messages in Go:

Import the required packages:

import (
    "mime/multipart"
    "net/mail"
)

Create a *multipart.Reader by passing the email or message body to multipart.NewReader:

msg, err := mail.ReadMessage(emailReader) // emailReader is of type io.Reader
if err != nil {
    // handle error
}

mimeReader := multipart.NewReader(msg.Body, msg.Header.Get("Content-Type"))

Iterate through the parts of the multipart message using the NextPart method of the multipart.Reader. This method returns the next part as a *multipart.Part:

for {
    part, err := mimeReader.NextPart()
    if err != nil {
        if err == io.EOF {
            break
        }
        // handle error
    }

    // Access the part's headers and body
    header := part.Header
    body, err := io.ReadAll(part)
    if err != nil {
        // handle error
    }

    // Do something with the part
    // e.g., save attachments to disk or process specific parts
}

Handle each part according to its content type or other relevant information from the headers. For example, save attachments to disk:

if header.Get("Content-Disposition") != "" {
    // Check if it's an attachment
    disposition, params, err := mime.ParseMediaType(header.Get("Content-Disposition"))
    if err != nil {
        // handle error
    }

    if disposition == "attachment" {
        // Extract the filename and save the attachment to disk
        filename := params["filename"]
        saveAttachment(body, filename)
    }
}

In the saveAttachment function, you can perform any custom logic to save the attachment to disk or process it according to your requirements.

Note that this example assumes you have an emailReader of type io.Reader that provides the email message content (e.g., from reading an email from a file or a network connection).