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_diag3_test.go

@@ -0,0 +1,67 @@
+package internal
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/teamwork/tnef"
+)
+
+func TestTNEFAllSizes(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)
+
+		totalAttrSize := 0
+		for _, attr := range decoded.Attributes {
+			totalAttrSize += len(attr.Data)
+			fmt.Printf("  Attr 0x%04X: %d bytes\n", attr.Name, len(attr.Data))
+		}
+
+		totalAttSize := 0
+		for _, ta := range decoded.Attachments {
+			totalAttSize += len(ta.Data)
+		}
+
+		fmt.Printf("\nTotal TNEF data: %d bytes\n", len(data))
+		fmt.Printf("Total attribute data: %d bytes\n", totalAttrSize)
+		fmt.Printf("Total attachment data: %d bytes\n", totalAttSize)
+		fmt.Printf("Accounted: %d bytes\n", totalAttrSize+totalAttSize)
+		fmt.Printf("Missing: %d bytes\n", len(data)-totalAttrSize-totalAttSize)
+
+		// Try raw decode to check for nested message/attachment objects
+		fmt.Printf("\nBody: %d, BodyHTML: %d\n", len(decoded.Body), len(decoded.BodyHTML))
+
+		// Check attachment[0] content
+		if len(decoded.Attachments) > 0 {
+			a0 := decoded.Attachments[0]
+			fmt.Printf("\nAttachment[0] Title=%q Data (hex): %x\n", a0.Title, a0.Data)
+		}
+	}
+}