Default Programs Editor is a powerful but simple file association utility for Windows. It is a context menu editor, an autoplay editor, and default programs association editor.
File Type Settings:
-Add, Edit, and Delete context menu items
-Edit file type information (such as icon and description)
-Change an extension’s associated file type
-Add, Edit, and Delete autoplay handler programs
-Change which autoplay options are available for any media type
-Change the default autoplay handler
Default Programs Settings:
-Add or Remove default associations (possible to check, but not un-check, in Windows)
-Disable the “Search web for unknown extension” dialog
-Standalone utility, or install as a Control Panel applet
-Make direct changes to the system registry, or export edits as .reg files
-Designed for Vista and Windows 7 with UAC support
Go here if you are unable to Change Default Program Extension on Windows 7.