AttachmentTempDirectory is a string property that indicates the path to use when creating or accessing the temporary files needed for file-based attachments in the message. AttachmentTempDirectory is used when TIdAttachmentFile instances are added to the MessageParts for the message. 

When assigning a new value to AttachmentTempDirectory, a trailing path delimiter in the new property value is removed. Changing the value in AttachmentTempDirectory after loading or creating file-based attachments does not affect the temp file path or stored path name used for the existing attachments. Assign the value for the AttachmentTempDirectory property prior to loading a message or manipulating the MessageParts in the message.

MessageParts 

TIdAttachmentFile.StoredPathName