Windows XP has the ability to create file type association for a new file extension in Control Panel, Folder Options, File Types tab. Unfortunately, the File Types tab is removed in Windows Vista and the Default Programs applet does not provide an option to create new file type association.
You can however use the Open With option to associate new or existing file types with applications of your choice.
Creating New file type association
1. Find a file with the extension that you wish to add
2. Right-click the file and choose Open
3. Click Select a program from a list of installed programs
4. Click OK
5. If the application is not listed in the Open With dialog, click Browse and locate the executable.
6. Make sure that the following checkbox is enabled:
7. Click OK.
File type association is now created for the new file type, but only for the current user account. If you have many user accounts in the system and you want to create a global file association setting, you can use the ASSOC and FTYPE commands. These commands need to be used from an elevated Command Prompt window.
To associate the file type .mx with c:\windows\mxviewer.exe, run the following commands from an elevated Command Prompt window.
This associates .mx file type with mxviewer.exe. To add other file class data, such as setting the default icon or customizing the file type description, you’ll have to use the Registry Editor or use one of the third-party alternatives mentioned below.
Alternatives to the File Types tab
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Ramesh Srinivasan founded Winhelponline.com back in 2005. He is passionate about Microsoft technologies and has a vast experience in the ITeS industry — delivering support for Microsoft's consumer products. He has been a Microsoft MVP [2003 to 2012] who contributes to various Windows support forums.