FieldObject is a TObject property that optionally contains an object reference with content values for the form data field. FieldObject normally contains a TStream descendant reflecting the contents of FileName, or a TStrings descendant with generic textual content. Assigning a TObject reference other than TStream or TStrings to the FieldObject property will raise an EIdInvalidObjectType exception. 

FieldObject is used when methods of TIdMultiPartFromDataStream, like AddObject and AddFile, are used to create form data fields. If AddObject is used (with an existing object reference), it is the applications responsibility for freeing the object reference. 

Use FieldStream to access the TStream created for a form data field that represents a file to include in the miultipart form data stream.

TIdFormDataField.FieldStream