In order to action attachments, you use the action button in the toolbar. WinDat Opener will then display a menu of possible actions. These may include:
The option to preview the file directly with WinDat Opener. WinDat Opener has the ability to preview many common file types, including spreadsheets, word processing and presentation formats.
The option to open the file in another App. If you have an App installed that can either view or edit the extracted file, the option to open the file in that App will appear.
Note that if WinDat Opener doesn't have ability to preview the file and there is no other App that can view or open it, no menu will appear.