Add new content types and refactor message writer

Introduced "multipart/mixed" and "multipart/related" content types in encoding.go and updated msgwriter.go to accommodate these. Adjustments made in related tests for these new types. Additionally, removed unnecessary print statements and improved multipart alternative parsing in eml.go.

eml.go

@@ -178,9 +178,6 @@ func parseEMLHeaders(mh *nm.Header, m *Msg) error {
 	for _, h := range commonHeaders {
 		if v := mh.Get(h.String()); v != "" {
 			m.SetGenHeader(h, v)
-			if strings.EqualFold(h.String(), "subject") {
-				fmt.Printf("SUBJECT: %s\n", m.GetGenHeader(HeaderSubject)[0])
-			}
 		}
 	}
 
@@ -204,7 +201,8 @@ func parseEMLBodyParts(pm *nm.Message, bodybuf *bytes.Buffer, m *Msg) error {
 		if err := parseEMLBodyPlain(mediatype, pm, bodybuf, m); err != nil {
 			return fmt.Errorf("failed to parse plain body: %w", err)
 		}
-	case strings.EqualFold(mediatype, TypeMultipartAlternative.String()):
+	case strings.EqualFold(mediatype, TypeMultipartAlternative.String()),
+		strings.EqualFold(mediatype, "multipart/mixed"):
 		if err := parseEMLMultipartAlternative(params, bodybuf, m); err != nil {
 			return fmt.Errorf("failed to parse multipart/alternative body: %w", err)
 		}
@@ -261,7 +259,6 @@ func parseEMLMultipartAlternative(params map[string]string, bodybuf *bytes.Buffe
 			_ = mpart.Close()
 			return fmt.Errorf("failed to read multipart: %w", err)
 		}
-		fmt.Printf("CTE: %+v", params)
 
 		mpContentType, ok := mpart.Header[HeaderContentType.String()]
 		if !ok {

encoding.go

@@ -140,6 +140,8 @@ const (
 const (
 	TypeAppOctetStream       ContentType = "application/octet-stream"
 	TypeMultipartAlternative ContentType = "multipart/alternative"
+	TypeMultipartMixed       ContentType = "multipart/mixed"
+	TypeMultipartRelated     ContentType = "multipart/related"
 	TypePGPSignature         ContentType = "application/pgp-signature"
 	TypePGPEncrypted         ContentType = "application/pgp-encrypted"
 	TypeTextHTML             ContentType = "text/html"

encoding_test.go

@@ -40,6 +40,18 @@ func TestContentType_String(t *testing.T) {
 			"ContentType: application/octet-stream", TypeAppOctetStream,
 			"application/octet-stream",
 		},
+		{
+			"ContentType: multipart/alternative", TypeMultipartAlternative,
+			"multipart/alternative",
+		},
+		{
+			"ContentType: multipart/mixed", TypeMultipartMixed,
+			"multipart/mixed",
+		},
+		{
+			"ContentType: multipart/related", TypeMultipartRelated,
+			"multipart/related",
+		},
 		{
 			"ContentType: application/pgp-signature", TypePGPSignature,
 			"application/pgp-signature",

msgwriter.go

@@ -89,11 +89,11 @@ func (mw *msgWriter) writeMsg(m *Msg) {
 	}
 
 	if m.hasMixed() {
-		mw.startMP("mixed", m.boundary)
+		mw.startMP(MIMEMixed, m.boundary)
 		mw.writeString(DoubleNewLine)
 	}
 	if m.hasRelated() {
-		mw.startMP("related", m.boundary)
+		mw.startMP(MIMERelated, m.boundary)
 		mw.writeString(DoubleNewLine)
 	}
 	if m.hasAlt() {