FieldSize is a LongInt property that indicates the size of the form data field in a multipart form data stream. FieldSize includes the MIME boundary marker from the multipart form data stream, and other HTTP headers needed to reflect the content type and disposition for an HTTP GET or POST request. 

When FieldObject has been assigned, FieldSize includes the length of any content assigned to the TStream buffer or TStrings lines with an additional CR+LF sequence to separate content values.

TIdFormDataField.ContentType 

TIdFormDataField.FileName 

TIdFormDataField.FieldObject 

TIdMultiPartFormDataStream.Boundary 

TIdMultiPartFormDataStream.RequestContentType