TipHKCU\Software\Microsoft\Windows NT\CurrentVersion\
The Extensions subkey stores entries which associate file name extensions with command line executions. These entries are used only when a file name extension is not already associated with a program.
Each of the entries in this subkey has the following format:
file name extension REG_SZ command line to execute
For example, the entry
txt REG_SZ notepad.exe^.txt
indicates that the default action for all files with the extension .txt is to open the file with notepad, whereas
txt REG_SZ notepad.exe^.txt -openandprint
indicates that the default action for all files with the extension .txt is to open the file with notepad and print it to the default printer.
The caret symbol (^) is replaced with the name of the selected file in the command line at run time.
To associate a file type with a program, use Windows Explorer. From the Tools menu, click Folder Options, and then click the File Types tab.
Tip
For more information about creating a file association, search the Platform Software Development Kit (SDK) documentation in the MSDN Library at http://msdn.microsoft.com.