CanonicalName: PidTagAttachNumber
PropertyID: $0E21
DataType: PT_LONG
Area: Message attachment
Description: Contains a number that uniquely identifies the attachment within its parent
Remarks: Message stores generate and maintain this property. The attachment number
is the secondary sort key, after the rendering position, in the attachment

PR_ATTACH_NUM is used to open the attachment with the IMessage::OpenAttach
method. Within a client application's session, the PR_ATTACH_NUM property
of a message attachment remains constant as long as the attachment table is

The message store propagates changes to the table using the
Imessage::CreateAttach and Imessage::DeleteAttach methods. At its option
the message store can generate table notifications on open attachment
tables so that clients can resynchronize to those changes.