This behaviour is new in KDE 4.8.1. When you select Create new --> Text file and enter a name like test.txt, the file will actually be named test.txt.desktop. When you enter a name without extension like test, the name will not be changed. The same applies for Create new --> HTML file. It happens on the desktop as well as in local folders.
*** This bug has been marked as a duplicate of bug 295490 ***