I give you the manual instructions.
Open My Computer.
Go to Tools > Folder Options
Select View tab.
Under Hidden files and folders, untick this box:
Hide extensions for known file types.
Click OK.
Close My Computer. Navigate to that file, you should be able to see
the whole file name, like whatever.dat
Change that .dat to the correct extension.
Go back to the Tools > Folder Options and tick back the box, if
not, you anyhow change things then jialat liao...
