MAPI look into tnef attachments to detect protocol
b8d651c4d083
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Description

MAPI look into tnef attachments to detect protocol

* src/mapihelp.cpp (mapi_get_first_attach_data): New.
(change_message_class_ipm_note_smime): Remove ms-tnef handling.
Add multipart/signed protocol support for smime.
Warn if encrypted fallback is used.
(change_message_class_ipm_note_smime_multipartsigned): Remove
ms-tnef handling.
(parse_header_data): New helper factored out form
(mapi_get_message_content_type): Handle ms-tnef wrapping.

If we have a mail with content-type ms-tnef we now
look into the tnef attachment to parse the protocol
for multipart/signed.

While Outlook makes the original content type available
through API (get_first_attach_mime_tag) it does not
provide the protocol.

As we need this information early on for display purposes
we now look into the attachment.