如何使用mxk/go-imap查找和下载附件？

import (
    "github.com/mxk/go-imap/imap"
)

client, err := imap.DialTLS("imap.example.com:993", nil)
if err != nil {
    log.Fatal(err)
}
defer client.Logout()

if err := client.Login("username", "password"); err != nil {
    log.Fatal(err)
}

mailbox := "INBOX"
if _, err := client.Select(mailbox, false); err != nil {
    log.Fatal(err)
}

criteria := imap.NewSearchCriteria()
criteria.WithoutFlags = []string{imap.SeenFlag}

uids, err := client.Search(criteria)
if err != nil {
    log.Fatal(err)
}

for _, uid := range uids {
    seqSet := new(imap.SeqSet)
    seqSet.AddNum(uid)

    messages := make(chan *imap.Message, 1)
    done := make(chan error, 1)
    go func() {
        done <- client.Fetch(seqSet, []imap.FetchItem{imap.FetchEnvelope, imap.FetchBodyStructure}, messages)
    }()

    for msg := range messages {
        for _, att := range msg.BodyStructure.Attachments {
            // 下载附件
            section := &imap.BodySectionName{Section: att.PartSpecifier()}
            r := imap.NewSectionReader(msg, section)
            data, err := ioutil.ReadAll(r)
            if err != nil {
                log.Fatal(err)
            }

            // 处理附件数据
            // ...
        }
    }

    if err := <-done; err != nil {
        log.Fatal(err)
    }
}