Email option to embed images as base64 instead of link (#32061)

ref: #15081
ref: #14037

Documentation: https://gitea.com/gitea/docs/pulls/69

# Example
Content:

![image](https://github.com/user-attachments/assets/e73ebfbe-e329-40f6-9c4a-f73832bbb181)
Result in Email:

![image](https://github.com/user-attachments/assets/55b7019f-e17a-46c3-a374-3b4769d5c2d6)
Result with source code:
(first image is external image, 2nd is now embedded)

![image](https://github.com/user-attachments/assets/8e2804a1-580f-4a69-adcb-cc5d16f7da81)

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/mailer/mail.go

@@ -6,16 +6,26 @@ package mailer
 
 import (
 	"bytes"
+	"context"
+	"encoding/base64"
+	"fmt"
 	"html/template"
+	"io"
 	"mime"
 	"regexp"
 	"strings"
 	texttmpl "text/template"
 
+	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/httplib"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/storage"
+	"code.gitea.io/gitea/modules/typesniffer"
 	sender_service "code.gitea.io/gitea/services/mailer/sender"
+
+	"golang.org/x/net/html"
 )
 
 const mailMaxSubjectRunes = 256 // There's no actual limit for subject in RFC 5322
@@ -44,6 +54,107 @@ func sanitizeSubject(subject string) string {
 	return mime.QEncoding.Encode("utf-8", string(runes))
 }
 
+type mailAttachmentBase64Embedder struct {
+	doer         *user_model.User
+	repo         *repo_model.Repository
+	maxSize      int64
+	estimateSize int64
+}
+
+func newMailAttachmentBase64Embedder(doer *user_model.User, repo *repo_model.Repository, maxSize int64) *mailAttachmentBase64Embedder {
+	return &mailAttachmentBase64Embedder{doer: doer, repo: repo, maxSize: maxSize}
+}
+
+func (b64embedder *mailAttachmentBase64Embedder) Base64InlineImages(ctx context.Context, body template.HTML) (template.HTML, error) {
+	doc, err := html.Parse(strings.NewReader(string(body)))
+	if err != nil {
+		return "", fmt.Errorf("html.Parse failed: %w", err)
+	}
+
+	b64embedder.estimateSize = int64(len(string(body)))
+
+	var processNode func(*html.Node)
+	processNode = func(n *html.Node) {
+		if n.Type == html.ElementNode {
+			if n.Data == "img" {
+				for i, attr := range n.Attr {
+					if attr.Key == "src" {
+						attachmentSrc := attr.Val
+						dataURI, err := b64embedder.AttachmentSrcToBase64DataURI(ctx, attachmentSrc)
+						if err != nil {
+							// Not an error, just skip. This is probably an image from outside the gitea instance.
+							log.Trace("Unable to embed attachment %q to mail body: %v", attachmentSrc, err)
+						} else {
+							n.Attr[i].Val = dataURI
+						}
+						break
+					}
+				}
+			}
+		}
+		for c := n.FirstChild; c != nil; c = c.NextSibling {
+			processNode(c)
+		}
+	}
+
+	processNode(doc)
+
+	var buf bytes.Buffer
+	err = html.Render(&buf, doc)
+	if err != nil {
+		return "", fmt.Errorf("html.Render failed: %w", err)
+	}
+	return template.HTML(buf.String()), nil
+}
+
+func (b64embedder *mailAttachmentBase64Embedder) AttachmentSrcToBase64DataURI(ctx context.Context, attachmentSrc string) (string, error) {
+	parsedSrc := httplib.ParseGiteaSiteURL(ctx, attachmentSrc)
+	var attachmentUUID string
+	if parsedSrc != nil {
+		var ok bool
+		attachmentUUID, ok = strings.CutPrefix(parsedSrc.RoutePath, "/attachments/")
+		if !ok {
+			attachmentUUID, ok = strings.CutPrefix(parsedSrc.RepoSubPath, "/attachments/")
+		}
+		if !ok {
+			return "", fmt.Errorf("not an attachment")
+		}
+	}
+	attachment, err := repo_model.GetAttachmentByUUID(ctx, attachmentUUID)
+	if err != nil {
+		return "", err
+	}
+
+	if attachment.RepoID != b64embedder.repo.ID {
+		return "", fmt.Errorf("attachment does not belong to the repository")
+	}
+	if attachment.Size+b64embedder.estimateSize > b64embedder.maxSize {
+		return "", fmt.Errorf("total embedded images exceed max limit")
+	}
+
+	fr, err := storage.Attachments.Open(attachment.RelativePath())
+	if err != nil {
+		return "", err
+	}
+	defer fr.Close()
+
+	lr := &io.LimitedReader{R: fr, N: b64embedder.maxSize + 1}
+	content, err := io.ReadAll(lr)
+	if err != nil {
+		return "", fmt.Errorf("LimitedReader ReadAll: %w", err)
+	}
+
+	mimeType := typesniffer.DetectContentType(content)
+	if !mimeType.IsImage() {
+		return "", fmt.Errorf("not an image")
+	}
+
+	encoded := base64.StdEncoding.EncodeToString(content)
+	dataURI := fmt.Sprintf("data:%s;base64,%s", mimeType.GetMimeType(), encoded)
+	b64embedder.estimateSize += int64(len(dataURI))
+	return dataURI, nil
+}
+
 func fromDisplayName(u *user_model.User) string {
 	if setting.MailService.FromDisplayNameFormatTemplate != nil {
 		var ctx bytes.Buffer