example:
set /p "FullPathNameData=Please enter the full path name of backup file for YourBackup: "
for /f "delims=" %%a in ("%FullPathNameData%") do set "Extension=%%~xa"
if /i "%Extension%"==".bak" (echo %FullPathNameData% is a BAK file) else %FullPathNameData% is NOT a BAK file