Why mp4 or mp4.ts from identical installation?

[Nick Payne]

Well both downloads I've done since installing 2.98 yesterday (on
Windows 10) have resulted in three output files in the download
directory after the download completed, all three files being
approximately the same size: the expected download with mp4 file
extension, a temp-nnnnn.mp4 file (nnnnn being a five digit number), and
a file with the hls.ts extension.