Total Commander

TC Media player help wrote:Save playlist: Save playlist in m3u or m3u8 (Unicode) format

For me the Save playlist dialog has a pre-filter only for *.m3u8 files.

This is intentional: Android DELETES(!) playlists with m3u extensions which point to things it cannot access, like http, https, plugins etc. You can enter name with m3u extension manually, though.