Extension is added even when template does not contain formatting variable for it

[XANi]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2020.06.16.1
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

youtube-dl https://www.youtube.com/watch\?v\=0qkLFgMJJmA -o /tmp/1.file -v      
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['https://www.youtube.com/watch?v=0qkLFgMJJmA', '-o', '/tmp/1.file', '-v']
[debug] Encodings: locale UTF-8, fs utf-8, out utf-8, pref UTF-8
[debug] youtube-dl version 2020.06.16.1
[debug] Python version 3.8.3 (CPython) - Linux-5.6.0-2-amd64-x86_64-with-glibc2.29
[debug] exe versions: avconv 4.3-2, avprobe 4.3-2, ffmpeg 4.3-2, ffprobe 4.3-2, phantomjs ., rtmpdump 2.4
[debug] Proxy map: {}
[youtube] 0qkLFgMJJmA: Downloading webpage
[debug] Default format spec: bestvideo+bestaudio/best
[debug] Invoking downloader on 'https://r4---sn-f5f7lne6.googlevideo.com/videoplayback?expire=1594771336&ei=KPMNX8yYIfGJv_IPrPar6A8&ip=85.222.111.150&id=o-AOI9vBlsJFh-y3fdkNTqAV55grpybj26EWKMjoOm7NtH&itag=248&aitags=133%2C134%2C135%2C136%2C137%2C160%2C242%2C243%2C244%2C247%2C248%2C278&source=youtube&requiressl=yes&mh=t1&mm=31%2C26&mn=sn-f5f7lne6%2Csn-4g5e6nz7&ms=au%2Conr&mv=m&mvi=4&pl=18&initcwndbps=1573750&vprv=1&mime=video%2Fwebm&gir=yes&clen=40218408&dur=209.880&lmt=1543024205244104&mt=1594749633&fvip=4&keepalive=yes&c=WEB&txp=5432432&sparams=expire%2Cei%2Cip%2Cid%2Caitags%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&sig=AOq0QJ8wRQIhAIUH_7L1bgtbvDIC48mBeYsZdCGHqr0y9Eb8GorNx7qSAiB6FWnqGIVGdBS_DIVFXOMIkbL7k3GrHS4JRQb5aKQ8IA%3D%3D&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps&lsig=AG3C_xAwRAIgPGMN_aUobPhfQRr9y0TYTAenkyPevUXcBvqfRnHjGAYCIEQwysmUL73ISjBaz77gyhhfle0udEWYuueItrgNeZr3&ratebypass=yes'
[download] Destination: /tmp/1.file.f248
[download] 100% of 38.36MiB in 00:01
[debug] Invoking downloader on 'https://r4---sn-f5f7lne6.googlevideo.com/videoplayback?expire=1594771336&ei=KPMNX8yYIfGJv_IPrPar6A8&ip=85.222.111.150&id=o-AOI9vBlsJFh-y3fdkNTqAV55grpybj26EWKMjoOm7NtH&itag=251&source=youtube&requiressl=yes&mh=t1&mm=31%2C26&mn=sn-f5f7lne6%2Csn-4g5e6nz7&ms=au%2Conr&mv=m&mvi=4&pl=18&initcwndbps=1573750&vprv=1&mime=audio%2Fwebm&gir=yes&clen=2592539&dur=209.901&lmt=1543026149477938&mt=1594749633&fvip=4&keepalive=yes&c=WEB&txp=5411222&sparams=expire%2Cei%2Cip%2Cid%2Citag%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&sig=AOq0QJ8wRQIhAK-u41yeUNrc5KHJhuIcqkaDCFzH6-r2x3GG6-0QZwJcAiABFXvrFackfLrD0A90Xuq2mjF5EgfdFSWwtBvF5p847g%3D%3D&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps&lsig=AG3C_xAwRAIgPGMN_aUobPhfQRr9y0TYTAenkyPevUXcBvqfRnHjGAYCIEQwysmUL73ISjBaz77gyhhfle0udEWYuueItrgNeZr3&ratebypass=yes'
[download] Destination: /tmp/1.file.f251
[download] 100% of 2.47MiB in 00:00
[ffmpeg] Merging formats into "/tmp/1.file.webm"
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i file:/tmp/1.file.f248 -i file:/tmp/1.file.f251 -c copy -map 0:v:0 -map 1:a:0 file:/tmp/1.file.temp.webm
Deleting original file /tmp/1.file.f248 (pass -k to keep)
Deleting original file /tmp/1.file.f251 (pass -k to keep)

Description

When using format string in -o that does not contain any output template variable, in this example -o /tmp/1.file, youtube-dl still adds the extension for the video if the extension does not match the downloaded format, for example if the downloaded container format is webm, the following interaction happens:

• -o /tmp/1.file.webm => /tmp/1.file.webm
• -o /tmp/1.file -> /tmp/file.webm
• -o /tmp/1.file.mp4 -> "/tmp/file.mp4.webm

which is... confusing to say the least and as far as I've searched, undocumented.

In my particular case I've generated a tmpfile name and passed it to youtube-dl for output but it just appended a file extension to it for no good reason and of course script calling it couldn't predict that.