Windows NT evaluates file extensions on the first three characters, therefore ‘filename.html’ is actually treated as ‘filename.htm’. The side affect is that if you use a command such as ‘del *.htm’ to delete all the ‘.htm’ files, you will also delete all ‘.html’ files as well. This functionality can be disabled by modifying the registry.
To turn this setting off, editing the key found below and set the value of ‘Win95TruncatedExtensions’ to ‘0’, to turn the feature back on, set the value back to ‘1’.
Key: [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem]
Value Name: Win95TruncatedExtensions
Data Type: REG_DWORD
Data: (0 = disable , 1 = enable)