Adjust output to "BM" (Berserker's Multiworld) to differentiate spoiler logs, roms and server data at a glance

Rename _multidata to .multidata and embed .multidata as MultiServer-openable file into windows registry

MultiServer.py

@@ -1065,7 +1065,7 @@ async def main(args: argparse.Namespace):
             import tkinter.filedialog
             root = tkinter.Tk()
             root.withdraw()
-            data_filename = tkinter.filedialog.askopenfilename(filetypes=(("Multiworld data", "*multidata"),))
+            data_filename = tkinter.filedialog.askopenfilename(filetypes=(("Multiworld data", "*.multidata"),))
 
         ctx.load(data_filename)