ExtraData is a TIdBytes property that represents the binary content for extra data used in FSP protocol exchanges. ExtraData generally represents values for the file size and last modification timestamp transmitted with commands that retrieve or store files on the file system for the remote FSP host. 

Values in ExtraData are used and updated in the SendCmd method on the FSP client. 

Please note that ExtraData will not contains values when the remote FSP host does not implement ExtraData headers. Use the Version method and the SystemAcceptsExtraData property in TIdFSP to determine if values are included in the packets exchanged with the remote FSP host.

TIdFSP.SendCmd 

TIdFSP.SendCmdOnce 

TIdFSP.Version 

TIdFSP.SystemAcceptsExtraData