Could you please share your sample message file with us and the complete sample code that you are executing at your end? Please try the MailMessageSaveOptions.None option while saving the MHT file to disc and let us know your feedback.
I forgot to change MailMessageSaveType with MailMessageSaveOptions...now it works fine.
But i cant apply PDF-Attachments sent from Outlook (msg-Files) via converting to PDF. The mime-type are always "application/octet-stream" instead of "application/pdf".