constructor Create(
    Collection: TIdCollection
); override; overload;

Create is an overloaded reintroduced constructor for the object instance. 

Create calls the inherited constructor using Collection as the owner for the memory-based message attachment. 

Create allocates the TIdMemoryStream instance used in the DataStream property. 

Use an overloaded variant of the constructor to load the contents of the memory-based attachment from a stream instance.