Solution:

1) Open Windows Explorer.
'Windows Explorer' on 'Start' menu
2) Select the 'View' menu and select 'Options...'. (The 'Options' dialog box appears.)
3) Click the 'File Types' tab.
4) Click 'New Type...'. (The 'Add New File Type' dialog box appears.)
'New Type'
5) Type a description of the file type in the 'Description of type' box.
'Add New File Type' dialog box
NOTE: The file type will be listed alphabetically by this description in the 'Registered file types' list box.
6) Type the new file extension in the 'Associated extension' box.
7) (Optional) Select the 'Enable Quick View' check box to be able to use the Quick View feature with the files associated with this new file type.
Files readable in Quick View
8) (Optional) Select the 'Always show extension' check box to display the file extension when viewing a file associated with it.
9) Click 'New...'. (The 'New Action' dialog box appears.)
10) Type a description in the 'Action' box.
EXAMPLE: Open a text file
NOTE: This entry appears as a selection on the file's pop-up menu when you right-click on it.
11) Click 'Browse...'. (The 'Open With' dialog box appears.)
'New Action' dialog box
12) Select the drive containing the application program file from the 'Look in' drop-down list box.
13) Double-click the folder containing the program file in the list box.
14) Select the program file from the list box.
15) Click 'Open'. (The 'New Action' dialog box reappears, with the application's path and filename in the 'Application used to perform action' box.)
16) Click 'OK'. (The 'Add New File Type' dialog box reappears, with the new action added to the 'Actions' list box.)
17) (Optional) To associate more than one action with a file type, repeat steps 9) to 16).
18) Click 'Close'. (The 'Options' dialog box reappears, with the new file type added to the 'Registered file types' list box.)
19) Click 'Close'.