A configuration option checkbox reads "Send OpenPGP mails without attachments as PGP/Inline".
However, even when this box is checked inline PGP is not used.
Inline PGP is a handy feature because it allows compatibility with Mailvelope. In fact, GgpOL is able to decrypt mailvelope-created messages fine. The reverse is not true, due to this bug.
Test environment is Outlook 2010.