Look at SetUserFileIO for details.

procedure ImSetUserFileIO(OpenReadProc: TOpenReadProc; 
                          OpenWriteProc: TOpenWriteProc; 
                          CloseProc: TCloseProc; 
                          EofProc: TEofProc; 
                          SeekProc: TSeekProc; 
                          TellProc: TTellProc; 
                          ReadProc: TReadProc; 
                          WriteProc: TWriteProc); cdecl;

ImagingExport (at line 194)