TIdAttachment.ContentDisposition?

Hello,

in Indy9 Documentation there is "TIdAttachment.ContentDisposition" but
in actual DevSnapshot of Indy9 can't access this property?

code:
--------------------------------------------------------------------

if FMailMessage.MessageParts.Count > 0 then
begin
  for i:= 0 to Pred(FMailMessage.MessageParts.Count) do
  begin
    if (FMailMessage.MessageParts.Items[i] is TIdAttachment) then
    begin
      FMailMessage.MessageParts.Items[i].ContentDisposition ???
...
--------------------------------------------------------------------

Where to find this property?
What i'm doing wrong?

Thanks
Danijel