9.29. Attach Plus problems if using in-line replies in Office 2013
If using in-line replies with Office 2013, the attach file button doesn't pop Attach Plus. Also, if launching Attach Plus from a Send-To command with an in-line reply in progress, Attach Plus will not find the draft as an available message on which to attach.
In a nutshell, Attach Plus is not compatible with in-line replies - So they must be disabled.
Disable in-line replies by going to the File > Options dialog, under the 'Mail' tab, check the "Open replies and forwards in a new window" box: