I am not sure I understand.  The author of a message is responsible for inserting a Content-Description but Pegasus Mail does the equivalent to this by inserting a Content-Type, along with its Type= sub-descriptor. ie

  Content-type: Application/Octet-stream; name="UserChoice.reg.unicode"; type=Unknown

Martin

There seems to be no way to add a description (Content-Description header field as in RFC2045) to an attachment.

 I think this would be an useful feature to have.

In fact, Pegasus already displays the description field in attachments, for incoming mail; it seems strange not to be able to write it as well.