@EXT[filename] : Returns the extension from filename, without a leading period. On volumes which support long file names, the extension can be up to 255 characters long. On FAT drives it can be up to 3 characters long. filename must be quoted if it contains white space or special characters.
On an LFN drive, the returned extension may contain white space or special characters. To avoid problems which could be caused by these characters, quote the returned extension before you pass it to other commands.
set COMSPEC="c:\program files\jpsoft\tcmd17x64\tcc.exe"
echo %@ext["LFN Names may have.very long extensions"]
very long extensions