This article was previously published under Q218805
When you double-click an unknown file type, an unexpected default action may occur.
When you right-click an unknown file type, the Open With command may be missing from the context menu that appears.
This behavior can occur if the registry key that handles the command is altered or damaged.
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows
Use Registry Editor to examine the file association key for unknown file types: