check your file view from uptop in my computer folderview section and make sure the extension is shown and then clcik the file you want to make a .dat file and change it
mir2.exe change to mir2.dat
or another way is place the file you want to rename into the main directory of C:\
then click start and select run in that box type CMD and press enter
in the dos window type c:\ or .. or whatever the command is to return you to C:\> and then type @rename mir2.exe mir2.dat