gpg: set a name for the attached pgp signature part

This makes it more explicit for non pgp compatible clients. Without
this, they may show "unnamed part" or "noname".

Signed-off-by: Robin Jarry <robin@jarry.cc>
Acked-by: Moritz Poldrack <moritz@poldrack.dev>

lib/crypto/gpg/writer.go

@@ -69,7 +69,7 @@ func (s *Signer) Close() (err error) {
 	s.w.Write([]byte("\r\n"))
 
 	var signedHeader textproto.Header
-	signedHeader.Set("Content-Type", "application/pgp-signature")
+	signedHeader.Set("Content-Type", "application/pgp-signature; name=\"signature.asc\"")
 	signatureWriter, err := s.mw.CreatePart(signedHeader)
 	if err != nil {
 		return err