JSI Tip 0014 - Short File Extensions.

There's a registry setting that makes 4+ character extensions look like 3 character extensions. NT 4.0 defaults to ON so that DEL *.htm will also delete *.HTML.
HKEY_LOCAL_MACHINE\system\currentcontrolset\control\filesystem
value: Win95TruncatedExtensions: REG_DWORD:
0 = off
1 = on

You must reboot for this setting to take effect.