China DOS Union

Below is a batch file for renaming files in bulk. I also want to achieve the following functions, please ask an expert for help:
1 Delete the generated NAME text file
2 If I don't know the file type, how can I determine it
3 Long filenames can't be renamed, I don't know how to solve that!

@echo off
setlocal enabledelayedexpansion
set /a num=0
dir *.jpg >>name.txt
for /f "skip=5 tokens=4" %%a in (name.txt) do (
set yum=%%a
set /a num+=1
set num=!num!.jpg
call :start %%a !num!
)
:start
ren %1 %2
goto eof

@echo off
setlocal enabledelayedexpansion
set /a num=0
set /p ext=Enter the extension of the files to be batch-renamed (for example: txt):
dir /b *.%ext% >>name.txt
for /f "tokens=*" %%a in (name.txt) do (
set /a num+=1
set num=!num!.%ext%
echo %%a--^>!num!
ren "%%a" "!num!"
)
echo Batch rename completed!
pause>nul
del /f /q name.txt