GODT-213: Force no transfer encoding for embedded message/rfc822 parts

pkg/message/build_framework_test.go

@@ -93,8 +93,9 @@ func addTestAttachment(
 		Name:     name,
 		MIMEType: mimeType,
 		Header: map[string][]string{
-			"Content-Type":        {mimeType},
-			"Content-Disposition": {disposition},
+			"Content-Type":              {mimeType},
+			"Content-Disposition":       {disposition},
+			"Content-Transfer-Encoding": {"base64"},
 		},
 		Disposition: disposition,
 		KeyPackets:  base64.StdEncoding.EncodeToString(enc.GetBinaryKeyPacket()),

pkg/message/build_rfc822.go

@@ -382,6 +382,8 @@ func getAttachmentPartHeader(att *pmapi.Attachment) message.Header {
 	// Use base64 for all attachments except embedded RFC822 messages.
 	if att.MIMEType != "message/rfc822" {
 		hdr.Set("Content-Transfer-Encoding", "base64")
+	} else {
+		hdr.Del("Content-Transfer-Encoding")
 	}
 
 	return hdr