.m3u8 is a file format for video streams. It works like a playlist, it's a file that defines a collection of individual video files, each being one small part of the stream.
m3u8's can't be played natively by a browser, and if the OP has enabled autoplay media, kbin is probably directing the browser to that URL, the browser can't play it in-tab, so it downloads it as a file instead.
Quirky little bug you found there OP. Codeberg would love to know.
MarkAndrew