Hola,
Hay dos instrucciones que son obligatorias antes del post y que te hacen falta. Concretamente el PrepareStreamForDispatch y ubicar la Position en 0.
Mira este ejemplo :
Código Delphi
[-]Var
TheURL: String;
ResponseStream: TMemoryStream;
MultiPartFormDataStream: TMsMultiPartFormDataStream;
Begin
....
TheURL := 'http://www.piolillosoft.com/bajar/bajar.php';
MultiPartFormDataStream := TMsMultiPartFormDataStream.Create;
ResponseStream := TMemoryStream.Create;
try
SCWHTTP.Request.ContentType := MultiPartFormDataStream.RequestContentType;
MultiPartFormDataStream.AddFormField('CODE', ACode);
MultiPartFormDataStream.AddFile('Archivo', AFileName, 'image/jpg');
MultiPartFormDataStream.PrepareStreamForDispatch;
MultiPartFormDataStream.Position := 0;
SCWHTTP.Post(TheURL,MultiPartFormDataStream, ResponseStream);
finally
MultiPartFormDataStream.Free;
ResponseStream.Free;
end;
End;