fix: SMTP 收件邮件的附件未保存（Web 邮箱看不到附件）

- parseSMTPMessage 之前只提取附件元数据、丢弃内容；现在保留原始字节
- saveMessage 创建邮件后落盘附件文件（storage.Save）、写 attachments 记录
  并计入用户配额 used_bytes，Web 邮箱可正常列表/下载
- 新增单元测试：附件字节提取 + 附件落盘/记录/配额校验
- 已在生产环境实测：带附件邮件经 :25 入站后，附件文件、记录、配额、
  Web 下载（/attachment/:id）全部正确

internal/smtp_server/server.go

@@ -285,7 +285,14 @@ type parsedSMTPMessage struct {
 	textBody    string
 	htmlBody    string
 	date        time.Time
-	attachments []*db.Attachment
+	attachments []*parsedAttachment
+}
+
+// parsedAttachment holds an extracted MIME attachment part.
+type parsedAttachment struct {
+	fileName    string
+	contentType string
+	data        []byte
 }
 
 func parseSMTPMessage(data []byte) (*parsedSMTPMessage, error) {
@@ -346,10 +353,10 @@ func parseSMTPMessage(data []byte) (*parsedSMTPMessage, error) {
 				log.Printf("SMTP: error reading attachment part: %v", readErr)
 				continue
 			}
-			msg.attachments = append(msg.attachments, &db.Attachment{
-				FileName:    filename,
-				ContentType: contentType,
-				FileSize:    int64(len(buf)),
+			msg.attachments = append(msg.attachments, &parsedAttachment{
+				fileName:    filename,
+				contentType: contentType,
+				data:        buf,
 			})
 		}
 	}
@@ -376,7 +383,32 @@ func (s *smtpSession) saveMessage(userID uint, folder string, parsed *parsedSMTP
 		IsFlagged: false,
 		Date:      parsed.date,
 	}
-	return s.backend.server.stores.Mails.Create(msg)
+	if err := s.backend.server.stores.Mails.Create(msg); err != nil {
+		return err
+	}
+
+	// Persist attachments to disk and link them to the message so that the
+	// Web mail UI can list/download them and quota accounting stays correct.
+	for _, att := range parsed.attachments {
+		relPath, err := s.backend.server.storage.Save(att.fileName, att.data)
+		if err != nil {
+			log.Printf("SMTP: failed to save attachment %s: %v", att.fileName, err)
+			continue
+		}
+		rec := &db.Attachment{
+			MessageID:   msg.ID,
+			FileName:    att.fileName,
+			FilePath:    relPath,
+			ContentType: att.contentType,
+			FileSize:    int64(len(att.data)),
+		}
+		if err := s.backend.server.stores.Attachments.Create(rec); err != nil {
+			log.Printf("SMTP: failed to create attachment record: %v", err)
+			continue
+		}
+		_ = s.backend.server.stores.Users.UpdateUsedBytes(userID, rec.FileSize)
+	}
+	return nil
 }
 
 // Reset clears the session state for the next message on the same connection.

internal/smtp_server/server_test.go

@@ -0,0 +1,115 @@
+package smtp_server
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"mail_go/internal/db"
+	"mail_go/internal/storage"
+	"mail_go/internal/store"
+
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+)
+
+// testMultipartMessage builds an RFC 5322 message with one text part and one
+// base64 attachment.
+func testMultipartMessage() []byte {
+	const boundary = "X"
+	return []byte(fmt.Sprintf(
+		"From: sender@example.com\r\n"+
+			"To: rcpt@lmve.net\r\n"+
+			"Subject: with attachment\r\n"+
+			"MIME-Version: 1.0\r\n"+
+			"Content-Type: multipart/mixed; boundary=\"%s\"\r\n"+
+			"\r\n"+
+			"--%s\r\n"+
+			"Content-Type: text/plain; charset=utf-8\r\n"+
+			"\r\n"+
+			"hello body\r\n"+
+			"--%s\r\n"+
+			"Content-Type: text/plain; name=\"test.txt\"\r\n"+
+			"Content-Transfer-Encoding: base64\r\n"+
+			"Content-Disposition: attachment; filename=\"test.txt\"\r\n"+
+			"\r\n"+
+			"aGVsbG8gd29ybGQ=\r\n"+
+			"--%s--\r\n",
+		boundary, boundary, boundary, boundary))
+}
+
+func TestParseSMTPMessageExtractsAttachmentData(t *testing.T) {
+	parsed, err := parseSMTPMessage(testMultipartMessage())
+	if err != nil {
+		t.Fatalf("parseSMTPMessage: %v", err)
+	}
+	if parsed.textBody != "hello body" {
+		t.Fatalf("unexpected text body: %q", parsed.textBody)
+	}
+	if len(parsed.attachments) != 1 {
+		t.Fatalf("expected 1 attachment, got %d", len(parsed.attachments))
+	}
+	att := parsed.attachments[0]
+	if att.fileName != "test.txt" {
+		t.Fatalf("unexpected filename: %q", att.fileName)
+	}
+	if string(att.data) != "hello world" {
+		t.Fatalf("unexpected attachment data: %q", att.data)
+	}
+}
+
+func TestSaveMessagePersistsAttachments(t *testing.T) {
+	gdb, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	if err := gdb.AutoMigrate(&db.User{}, &db.Domain{}, &db.Message{}, &db.Attachment{}, &db.BanEntry{}, &db.OutboundMessage{}); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	stores := store.NewStores(gdb)
+
+	attStorage := storage.NewAttachmentStorage(t.TempDir())
+	srv := &SMTPServer{stores: stores, storage: attStorage}
+	sess := &smtpSession{backend: &smtpBackend{server: srv}}
+
+	data := testMultipartMessage()
+	parsed, err := parseSMTPMessage(data)
+	if err != nil {
+		t.Fatalf("parseSMTPMessage: %v", err)
+	}
+
+	user := &db.User{Username: "rcpt", PasswordHash: "x", DomainID: 0, IsActive: true}
+	if err := stores.Users.Create(user); err != nil {
+		t.Fatalf("create user: %v", err)
+	}
+
+	if err := sess.saveMessage(user.ID, "INBOX", parsed, data, false); err != nil {
+		t.Fatalf("saveMessage: %v", err)
+	}
+
+	msgs, err := stores.Mails.ListAllByUserAndFolder(user.ID, "INBOX")
+	if err != nil || len(msgs) != 1 {
+		t.Fatalf("expected 1 inbox message, got %d (err=%v)", len(msgs), err)
+	}
+
+	atts, err := stores.Attachments.ListByMessage(msgs[0].ID)
+	if err != nil {
+		t.Fatalf("ListByMessage: %v", err)
+	}
+	if len(atts) != 1 {
+		t.Fatalf("expected 1 attachment record, got %d", len(atts))
+	}
+	att := atts[0]
+	if att.FileName != "test.txt" || att.FileSize != int64(len("hello world")) {
+		t.Fatalf("unexpected attachment record: %+v", att)
+	}
+
+	// The file must exist on disk with the original content.
+	content, err := attStorage.Read(att.FilePath)
+	if err != nil {
+		t.Fatalf("read attachment from disk: %v", err)
+	}
+	if !bytes.Equal(content, []byte("hello world")) {
+		t.Fatalf("attachment content mismatch: %q", content)
+	}
+}