Q:

How to upload an attachments in Go using these package "google.golang.org/appengine/mail"

Below is my code that I am trying to send with an attachment.
    msg := &mail.Message{
                    Sender: "kasireddy002@gmail.com",
                    To:     []string{addr},
          Attachments : []Attachment{
                           Name :"file name",
                            Data :[]byte,
                        ContentID :"fileid",
                      },

                    Subject: "Welcome to Simplyst Health: Verify your account",
    if err := mail.Send(context, msg); err != nil {
                    log.Errorf(ctx, "Alas, my user, the email failed to sendeth: err)
                }

When I am trying to save my code it is throwing an error.
ERROR:
cannot use []Attachment literal (type []Attachment) as type []"google.golang.org/appengine/mail".Attachment in field value

A:

You just need to change it to
msg := &mail.Message{
    Sender: "kasireddy002@gmail.com",
    To:     []string{addr},
    Attachments: []mail.Attachment{
        {
            Name:      "file name",
            Data:      []byte{},
            ContentID: "fileid",
        },
    },
    Subject: "Welcome to Simplyst Health: Verify your account",
}

Just to point out the issues with your code:

There was an error check within the definition of your mail.Message
The Attachment type was missing the package name, mail
As you were creating a slice of Attachments and not just one attachment, you needed to add extra {} around the attachment you wanted to add

