PRFile subclass on Gemstone Pier

Hi, i work with the Gemstone version of Pier 2.0. I use PRFile to save data into OODB but i'm interested to save some Files directly into directory on the server. For this i create a subclass of PRFile named PRFileOnDisk. In this class i define the method: isAbstract ^false label: ^ 'FIle on disk'. The fileModel class method return: ^ ISTMAExternalFileModel and the descriptionFile is set to: ^ MAFileDescription new kind: ISTMAExternalFileModel; accessor: #file; priority: 300; label: 'File on Disk'; beEditable; yourself The ISTMAExternalFileModel is a subclass of MAExternalFileModel and it work fine ( create directory and save data on the server disk ) for other Magritte description based on MAFileDescription ( with kind set to ISTMAExternalFileModel ). Now when in Pier i define a link with Type: 'File on disk' ( based on PRFileOnDisk ) the system create the subdirectory on the server disk but don't save the file on it . The file is managed how MAMemoryFileModel. Any idea about it ? Thanks, Dario