Add TNEF handling and email loading improvements

- Implement TNEF extraction and recursive parsing in new `tnef_reader.go` and associated tests.
- Create tests for TNEF extraction scenarios in `tnef_diag_test.go`, `tnef_diag7_test.go`, and `tnef_diag8_test.go`.

backend/utils/mail/tnef_reader_test.go

@@ -0,0 +1,59 @@
+package internal
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+func TestReadEmlWithTNEF(t *testing.T) {
+	testFile := `H:\Dev\Gits\EMLy\EML_TNEF.eml`
+	if _, err := os.Stat(testFile); os.IsNotExist(err) {
+		t.Skip("test EML file not present")
+	}
+
+	// First try the PEC reader (this is a PEC email)
+	email, err := ReadPecInnerEml(testFile)
+	if err != nil {
+		t.Fatalf("ReadPecInnerEml failed: %v", err)
+	}
+
+	fmt.Printf("Subject: %s\n", email.Subject)
+	fmt.Printf("From: %s\n", email.From)
+	fmt.Printf("Attachment count: %d\n", len(email.Attachments))
+
+	hasWinmailDat := false
+	for i, att := range email.Attachments {
+		fmt.Printf("  [%d] %s (%s, %d bytes)\n", i, att.Filename, att.ContentType, len(att.Data))
+		if att.Filename == "winmail.dat" {
+			hasWinmailDat = true
+		}
+	}
+
+	if hasWinmailDat {
+		t.Error("winmail.dat should have been expanded into its contained attachments")
+	}
+
+	if len(email.Attachments) == 0 {
+		t.Error("expected at least one attachment after TNEF expansion")
+	}
+}
+
+func TestReadEmlFallback(t *testing.T) {
+	testFile := `H:\Dev\Gits\EMLy\EML_TNEF.eml`
+	if _, err := os.Stat(testFile); os.IsNotExist(err) {
+		t.Skip("test EML file not present")
+	}
+
+	// Also verify the plain EML reader path
+	email, err := ReadEmlFile(testFile)
+	if err != nil {
+		t.Fatalf("ReadEmlFile failed: %v", err)
+	}
+
+	fmt.Printf("[EML] Subject: %s\n", email.Subject)
+	fmt.Printf("[EML] Attachment count: %d\n", len(email.Attachments))
+	for i, att := range email.Attachments {
+		fmt.Printf("  [%d] %s (%s, %d bytes)\n", i, att.Filename, att.ContentType, len(att.Data))
+	}
+}