feat(GODT-3193): assume text content type on attachments.

pkg/message/parser.go

@@ -552,6 +552,11 @@ func parseAttachment(h message.Header, body []byte) (Attachment, error) {
 	att.Header = mimeHeader
 	mimeType, mimeTypeParams, err := pmmime.ParseMediaType(h.Get("Content-Type"))
 
+	if err == pmmime.EmptyContentTypeErr {
+		mimeType = "text/plain"
+		err = nil
+	}
+
 	if err != nil {
 		return Attachment{}, err
 	}

pkg/mime/encoding.go

@@ -35,6 +35,8 @@ import (
 	"golang.org/x/text/encoding/htmlindex"
 )
 
+var EmptyContentTypeErr = errors.New("empty content type")
+
 func init() {
 	rfc822.ParseMediaType = ParseMediaType
 	proton.CharsetReader = CharsetReader
@@ -257,7 +259,7 @@ func DecodeCharset(original []byte, contentType string) ([]byte, error) {
 // ParseMediaType from MIME doesn't support RFC2231 for non asci / utf8 encodings so we have to pre-parse it.
 func ParseMediaType(v string) (string, map[string]string, error) {
 	if v == "" {
-		return "", nil, errors.New("empty media type")
+		return "", nil, EmptyContentTypeErr
 	}
 	decoded, err := DecodeHeader(v)
 	if err != nil {