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Cannot open file after renaming
If you cannot open a file after you have renamed it then you would have seen a warning informing you that changing the file type may make the file unusable and you have clicked "YES"
Windows uses the file type to work out which piece of software/program to use when opening a file, it now doesn't know what software to use as it has lost the file type
To prevent this happening again hide filetypes by doing the following:-
Open "My Documents", "My Pictures" or "My Computer" (or another folder)
Click "Tools" --> "Folder Options"
(If you cannot see tools at the top in the Menu Bar then press the ALT key on your keyboard once and it should appear)
This will display
Click the "View" tab at the top
Now scroll down and check the box "Hide extensions for known Filetypes"