By default, Windows stops users from viewing file extensions that have an associated program, but there are many occasions when we need to view a file's extension.
Luckily, it's an easy process and the setting can be changed in a few clicks of the mouse.
Open 'My Computer' or 'Windows Explorer' and select the 'Tools' item from the top menu bar and select 'Folder Options...' (fig a)
A new window should appear ('Folder Options...'). Click the 'View' tab from the top menu (fig b)
In the 'View' section, around half-way down the list of options, un-check the 'Hide extensions for known file types' option (fig c)