-Open the folder where the maps are located
-Shift right click on the window background to bring the context menu
-Click on open command window here
-In the command window paste this line for /f "Tokens=*" %f in ('dir /l/b/a-d *.pms') do (rename "%f" "%f")
-Hit enter
-Refresh your folder window to see the changes
Remember to back up your folder before.
Thanks to urraka for the info!