auto Best Quality not working

[vis15]

On youtube when there is a size greater then 1080p the best quality auto-detect does not see it. For example on this video: http://http://www.youtube.com/watch?v=9dgSa4wmMzk. The auto-detect chooses the 1080p version instead of the 4K version.

[cnbeining]

Same issue.

youtube-dl is not downloading the best quality video, even if I had set it to download the 2K one.

Example URL:
https://www.youtube.com/watch?v=i5CoN6tBP8w

Tried the userscript YouTube Links, and got funny result:

It can detect the 2K M4V URL, but the download would stop at 14M. The video itself cannot be played in the Flash player, and YouTube Links cannot find the audio link.

Example 2K URL found:

http://r15---sn-vgqsenel.googlevideo.com/videoplayback?algorithm=throttle-factor&burst=40&signature=391D072D77FF4E88D0FDAB18EBB9BA2CC488F83A.8445B6BB6A3E4E19B8CA9145B336D58F339F20FD&sver=3&key=yt5&ip=(*deleted on purpose*)&ipbits=0&upn=D3C4scmHvVA&factor=1.25&id=8b90a837ab413fcc&dur=212.200&gir=yes&lmt=1371070393235282&clen=128095994&mv=m&mt=1391072669&ms=au&source=youtube&itag=138&expire=1391099340&fexp=900225%2C939100%2C916623%2C936910%2C936913&title=%25E3%2580%25902K%25E7%2594%25BB%25E8%25B3%25AA%25E3%2580%2591%25E5%25B9%25BB%25E6%2583%25B3%25E4%25B8%2587%25E8%258F%25AF%25E9%258F%25A1PV%25E3%2580%258C%25E8%258F%25AF%25E9%25B3%25A5%25E9%25A2%25A8%25E6%259C%2588%25E3%2580%258D2048%25C3%25971152'

[jaimeMF]

Basically the problem is that for that resolutions, the file we can get has only the video, the audio is downloaded in another file (this is called DASH, just search on the bug tracker for more info).
You can force to download it using youtube-dl http://www.youtube.com/watch?v=9dgSa4wmMzk -f 138 (you can see all the formats available with the -F option). This would only download the video, but in the last versions of youtube-dl you can automatically download and merge (requires ffmpeg/avconv) the audio and video so you can play it in most players, you just need to run youtube-dl 'https://www.youtube.com/watch?v=9dgSa4wmMzk' -f '138+bestaudio'.
Note: This feature is not documented for good reasons, there are some things that need to get improved and maybe some things will need to get fixed.

[clampak]

This is now documented, isn't it? Can this be closed?

[vis15]

Yes you can close it.

[clampak]

Someone can, I don't have rights.