well if you look at it in windows exlorer then its a .m3u
No. m3u8 and m3u are different.
maybe because it uses UTF-8 encoding?
and exactly this is the difference. m3u uses any encoding (it could be utf-8 too). On windows most programs will assume m3u to use the local encoding, hence all filenames with non-ASCII characters won't work. Using m3u8 indicates that it is utf-8 and programs that know the difference also know how to handle the files correctly.
Apart from the encoding both formats are identical, thus you won't see a difference if your filenames only contain ASCII character.