How to add image to html body of an email (Go)

I am using Gomail

I am attempting to send an email to myself. I have figured out how to do so, but now I want to add an image, either in the html body or as an attachment (does not really matter). I just need to be able to see the image in my email.

First off, I am sending the img src through a POST request, which I then save into the following struct...

type test_struct struct {
    Test string `json:"image"`
}

I then try to attach it in the email body like so...

mail := gomail.NewMessage()
mail.SetHeader("From", "XXXX@gmail.com")
mail.SetHeader("To", "XXXX@gmail.com")
mail.SetHeader("Subject", "IMAGE!")

mail.SetBody("text/html", `<img src="cid:t.Test" alt="My Image"/>`)

this did not work, so I tried to use Embed method...

mail.Embed(t.Test)

This gave me a blank image with the error the filename or extension is too long. Not sure if this is important, but note that the src I got is from HTML5 Canvas. I got the source of the image from Canvas...

localCanvas.toDataURL('image/png');

And this is the value I save in t.Test...

buf, err := ioutil.ReadAll(req.Body)
reader := bytes.NewReader(buf)

var t test_struct

err = json.NewDecoder(reader).Decode(&t)

The string being save in the t.Test is of the following format...

data:image/png:base64, iVBORw0KGgoAAAANSUhEUgAAAoAAAAHgCAYAAAA10dzkAAAgA...

How can I send this to my email? The documentation only gets me so far.

First of all, check if the data in t.Test is correct (if contains the filename, path, or whatever you need to reference the image)

Well, you have some options, first, if it's ok just attaching the image, you only should do:

mail.Attach("/path/of/your/image/image.png")

But, if you need to use the image in the message, you have some alternatives.

Alt 1: Use Embed method

You must to have the image in your filesystem, because the method is defined in that way. You have to do something like:

mail.SetBody("text/html", "Image <img src=\"cid:tux.png\"/>")
mail.Embed("/path/of/your/image/tux.png")

You must to use the image name, in this case tux.png, when you add it in the html body with the img tag, you have to define the src like cid:tux.png

Alt 2: Use a base64 encoding

For this you need to know the mime type of the image file, in my example is a png file, so the mime is image/png. Also, you have to encode the image as base64.

img, err := ioutil.ReadFile("/path/of/your/image/tux.png")
if err != nil {
    // check errors
}
out := base64.StdEncoding.EncodeToString(img)

Or, if you don't have the image in the filesystem, but you have it in a variable ([]byte type), you can use it with EncodeToString

var imageData []byte
// in some part, you load the image data into imageData
out := base64.StdEncoding.EncodeToString(imageData)

And you use it like

mail.SetBody("text/html", fmt.Sprintf("Image <img src=\"data:image/png;base64,%s\"/>", out))

So, when you use img, you have to set in src an embedded image with data:mime_type;base64,image_encoded_base64, if we look the example, is data:image/png;base64,iVBORw0K...

EDIT:

I created an example with the JSON request and the handler Example