[mu4e rfe] tnef support

[wwlytton]

Is your feature request related to a problem? Please describe. I can't open attachments sent to me via microsoft tnef (application/ms-tnef; winmail.dat]...)

Describe the solution you'd like detach attachments; in some cases main email text is there in some cases hidden so would like to read

Describe alternatives you've considered tnef app in linux can be used

Additional context Add any other context or screenshots about the feature request here.

[djcb]

Does the tnef program work for your attachments? If so, you should be be able to use it from mu4e as well.