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_diag2_test.go

@@ -0,0 +1,58 @@
+package internal
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/teamwork/tnef"
+)
+
+func TestTNEFAttributes(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")
+	}
+
+	f, _ := os.Open(testFile)
+	defer f.Close()
+
+	outerEmail, _ := Parse(f)
+	var innerData []byte
+	for _, att := range outerEmail.Attachments {
+		if strings.Contains(strings.ToLower(att.Filename), "postacert.eml") {
+			innerData, _ = io.ReadAll(att.Data)
+			break
+		}
+	}
+
+	innerEmail, _ := Parse(bytes.NewReader(innerData))
+	for _, att := range innerEmail.Attachments {
+		data, _ := io.ReadAll(att.Data)
+		if strings.ToLower(att.Filename) != "winmail.dat" {
+			continue
+		}
+
+		decoded, _ := tnef.Decode(data)
+		fmt.Printf("MAPI Attributes (%d):\n", len(decoded.Attributes))
+		for _, attr := range decoded.Attributes {
+			dataPreview := fmt.Sprintf("%d bytes", len(attr.Data))
+			if len(attr.Data) < 200 {
+				dataPreview = fmt.Sprintf("%q", attr.Data)
+			}
+			fmt.Printf("  Name=0x%04X  Data=%s\n", attr.Name, dataPreview)
+		}
+
+		// Check Body/BodyHTML from TNEF data struct fields
+		fmt.Printf("\nBody len: %d\n", len(decoded.Body))
+		fmt.Printf("BodyHTML len: %d\n", len(decoded.BodyHTML))
+
+		// Check attachment details
+		for i, ta := range decoded.Attachments {
+			fmt.Printf("Attachment[%d]: title=%q dataLen=%d\n", i, ta.Title, len(ta.Data))
+		}
+	}
+}