ExtractFileExt

var
  fullFileName : string;

begin
  // Set up a full file name with drive and path
  fullFileName := 'C:\Program Files\Borland\Delphi7\Projects\Unit1.dcu';

  // Show the component parts of this full name
  ShowMessage('Drive = '+ExtractFileDrive (fullFileName));
  ShowMessage('Dir   = '+ExtractFileDir   (fullFileName));
  ShowMessage('Path  = '+ExtractFilePath  (fullFileName));
  ShowMessage('Name  = '+ExtractFileName  (fullFileName));
  ShowMessage('Ext   = '+ExtractFileExt   (fullFileName));
end;