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/format_detector.go

@@ -0,0 +1,47 @@
+package internal
+
+import (
+	"bytes"
+	"os"
+)
+
+// EmailFormat represents the detected format of an email file.
+type EmailFormat string
+
+const (
+	FormatEML     EmailFormat = "eml"
+	FormatMSG     EmailFormat = "msg"
+	FormatUnknown EmailFormat = "unknown"
+)
+
+// msgMagic is the OLE2/CFB compound file header signature used by .msg files.
+var msgMagic = []byte{0xD0, 0xCF, 0x11, 0xE0, 0xA1, 0xB1, 0x1A, 0xE1}
+
+// DetectEmailFormat identifies the email file format by inspecting the file's
+// binary magic bytes, regardless of the file extension.
+//
+// Supported formats:
+//   - "msg": Microsoft Outlook MSG (OLE2/CFB compound file)
+//   - "eml": Standard MIME email (RFC 5322)
+//   - "unknown": Could not determine format
+func DetectEmailFormat(filePath string) (EmailFormat, error) {
+	f, err := os.Open(filePath)
+	if err != nil {
+		return FormatUnknown, err
+	}
+	defer f.Close()
+
+	buf := make([]byte, 8)
+	n, err := f.Read(buf)
+	if err != nil || n < 1 {
+		return FormatUnknown, nil
+	}
+
+	// MSG files start with the OLE2 Compound File Binary magic bytes.
+	if n >= 8 && bytes.Equal(buf[:8], msgMagic) {
+		return FormatMSG, nil
+	}
+
+	// EML files are plain-text MIME messages; assume EML for anything else.
+	return FormatEML, nil
+}