%(ext)s broken in recent versions?

[SebiderSushi]

• I've verified and I assure that I'm running youtube-dl 2017.10.01

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Description of your issue, suggested solution and other information

I'm using -o '%(ext)s/%(upload_date)s-%(title)s-%(id)s.%(ext)s' to group all files by format but in recent versions this seems to be broken somehow - all files are written into the same directory.

Current Output:
youtube-dl -o '%(ext)s/%(upload_date)s-%(title)s-%(id)s.%(ext)s' --write-thumbnail https://www.youtube.com/watch?v=afvnzFEIz4s
[...]
[youtube] afvnzFEIz4s: Writing thumbnail to: mp4/ 20170929-StarStarSpace #21 - Relativistischer Dopplereffekt-afvnzFEIz4s .jpg
[...]
[download] Destination: mp4/20170929-StarStarSpace #21 - Relativistischer Dopplereffekt-afvnzFEIz4s.f137.mp4

What I'd expect and what was happening before:
[youtube] afvnzFEIz4s: Writing thumbnail to: jpg/ 20170929-StarStarSpace #21 - Relativistischer Dopplereffekt-afvnzFEIz4s .jpg

[dstftw]

ext (string): Video filename extension
Video

[dstftw]

Again, ext is video extension and the behaviour was always the same. No such "old behavior" you are claiming ever existed.

Also if this was consistent it should do this:
[youtube] afvnzFEIz4s: Writing thumbnail to: mp4 /20170929-StarStarSpace #21 - Relativistischer Dopplereffekt-afvnzFEIz4s .mp4

No, it's not. Otherwise it will overwrite video with thumbnail under mp4 ext.

[SebiderSushi]

Sorry, I was very sure that it did behave this way but I must've gotten confused somehow with my shellscripts to believe that this behaviour has ever been part of youtube-dl. I guess I have to apologize for being so stubborn about it.

To correct myself:
I would find it very useful if there was some attribute with the extension of the current file, so I could group my files into folders without any hassle.