setlocal EnableDelayedExpansion
set filename=file
set counter=0
for /f "usebackq delims=*" %%i in (`dir /b *.jpg`) do (
    set /a counter+=1
    ren "%%i" "%filename%!counter!.jpg"
)

setlocal EnableDelayedExpansion
set filename=file
set counter=0
for /f "usebackq delims=*" %%i in (`dir /b *.jpg`) do (
    set /a counter+=1
    ren "%%i" "%filename%!counter!.jpg"
)