added content type, mime type and content disposition for S/MIME singing purpose

encoding.go

@@ -19,6 +19,9 @@ type MIMEVersion string
 // MIMEType represents the MIME type for the mail
 type MIMEType string
 
+// Disposition represents a content disposition for the Msg
+type Disposition string
+
 // List of supported encodings
 const (
 	// EncodingB64 represents the Base64 encoding as specified in RFC 2045.
@@ -149,6 +152,7 @@ const (
 	TypePGPEncrypted         ContentType = "application/pgp-encrypted"
 	TypeTextHTML             ContentType = "text/html"
 	TypeTextPlain            ContentType = "text/plain"
+	typeSMimeSigned          ContentType = `application/pkcs7-signature; name="smime.p7s"`
 )
 
 // List of MIMETypes
@@ -156,6 +160,12 @@ const (
 	MIMEAlternative MIMEType = "alternative"
 	MIMEMixed       MIMEType = "mixed"
 	MIMERelated     MIMEType = "related"
+	MIMESMime       MIMEType = `signed; protocol="application/pkcs7-signature"; micalg=sha256`
+)
+
+// List of common content disposition
+const (
+	DispositionSMime Disposition = `attachment; filename="smime.p7s"`
 )
 
 // String is a standard method to convert an Charset into a printable format
@@ -172,3 +182,8 @@ func (c ContentType) String() string {
 func (e Encoding) String() string {
 	return string(e)
 }
+
+// String is a standard method to convert an Disposition into a printable format
+func (d Disposition) String() string {
+	return string(d)
+}

encoding_test.go

@@ -61,6 +61,11 @@ func TestContentType_String(t *testing.T) {
 			"ContentType: application/pgp-encrypted", TypePGPEncrypted,
 			"application/pgp-encrypted",
 		},
+
+		{
+			"ContentType: pkcs7-signature", typeSMimeSigned,
+			`application/pkcs7-signature; name="smime.p7s"; smime-type=signed-data`,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -121,3 +126,22 @@ func TestCharset_String(t *testing.T) {
 		})
 	}
 }
+
+// TestDisposition_String tests the string method of the Disposition object
+func TestDisposition_String(t *testing.T) {
+	tests := []struct {
+		name string
+		d    Disposition
+		want string
+	}{
+		{"Disposition: S/Mime", DispositionSMime, `attachment; filename="smime.p7s"`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.d.String() != tt.want {
+				t.Errorf("wrong string for Disposition returned. Expected: %s, got: %s",
+					tt.want, tt.d.String())
+			}
+		})
+	}
+}