--merge-output-format ogg doesn’t create a file with the extension *.ogg on the output

[mcepl]

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• I've verified and I assure that I'm running youtube-dl 2017.08.23

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• Searched the bugtracker for similar issues including closed ones

There is a possible duplicate at #5446, but I am not sure about what exactly it means.

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

Not sure whether it is a bug or not.

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If the purpose of this issue is a bug report, site support request or you are not completely sure provide the full verbose output as follows:

Add the -v flag to your command line you run youtube-dl with (youtube-dl -v <your command line>), copy the whole output and insert it here. It should look similar to one below (replace it with your log inserted between triple ```):

matej@mitmanek: ~$ youtube-dl --verbose --xattrs -x --merge-output-format ogg 'https://www.youtube.com/watch?v=w6EIFD80f90'
[debug] System config: [u'--prefer-free-formats']
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'--verbose', u'--xattrs', u'-x', u'--merge-output-format', u'ogg', u'https://www.youtube.com/watch?v=w6EIFD80f90']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.08.23
[debug] Python version 2.7.5 - Linux-3.10.0-693.1.1.el7.x86_64-x86_64-with-redhat-7.4-Maipo
[debug] exe versions: ffmpeg 2.8.12, ffprobe 2.8.12
[debug] Proxy map: {}
[youtube] w6EIFD80f90: Downloading webpage
[youtube] w6EIFD80f90: Downloading video info webpage
[youtube] w6EIFD80f90: Extracting video information
[youtube] w6EIFD80f90: Downloading MPD manifest
[debug] Invoking downloader on u'https://r7---sn-vufvj1-2gbe.googlevideo.com/videoplayback?ms=au&itag=251&initcwndbps=1205000&mv=m&mt=1503683683&sparams=clen%2Cdur%2Cei%2Cgcr%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cexpire&ei=qmSgWZ6iMsbuW7KMhKAF&keepalive=yes&mn=sn-vufvj1-2gbe&mm=31&clen=3286147&expire=1503705354&pl=21&mime=audio%2Fwebm&lmt=1431015871279088&dur=216.881&source=youtube&key=yt6&ipbits=0&gcr=cz&id=o-AEJvG5ByyjZ5Go2LdRND11SUHfcE5d0RUSTIjCrXp_4G&signature=DD2B9C9D93121BA75AC52C0D7BC97C136ABF2439.621AA1A5456C83C2CF2B677A2404BB800D07C295&gir=yes&requiressl=yes&ip=83.208.252.159&ratebypass=yes'
[download] Destination: The Unthanks - Magpie - Later… with Jools Holland - BBC Two-w6EIFD80f90.webm
[download] 100% of 3.13MiB in 00:01
[debug] ffmpeg command line: ffprobe -show_streams 'file:The Unthanks - Magpie - Later… with Jools Holland - BBC Two-w6EIFD80f90.webm'
[ffmpeg] Destination: The Unthanks - Magpie - Later… with Jools Holland - BBC Two-w6EIFD80f90.opus
[debug] ffmpeg command line: ffmpeg -y -i 'file:The Unthanks - Magpie - Later… with Jools Holland - BBC Two-w6EIFD80f90.webm' -vn -acodec copy 'file:The Unthanks - Magpie - Later… with Jools Holland - BBC Two-w6EIFD80f90.opus'
Deleting original file The Unthanks - Magpie - Later… with Jools Holland - BBC Two-w6EIFD80f90.webm (pass -k to keep)
[metadata] Writing metadata to file's xattrs
matej@mitmanek: ~$ ffplay -stats -vn -an The\ Unthanks\ -\ Magpie\ -\ Later…\ with\ Jools\ Holland\ -\ BBC\ Two-w6EIFD80f90.opus 
ffplay version 2.8.12 Copyright (c) 2003-2017 the FFmpeg developers
  built with gcc 4.8.5 (GCC) 20150623 (Red Hat 4.8.5-11)
  configuration: --prefix=/usr --bindir=/usr/bin --datadir=/usr/share/ffmpeg --incdir=/usr/include/ffmpeg --libdir=/usr/lib64 --mandir=/usr/share/man --arch=x86_64 --optflags='-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic' --extra-ldflags='-Wl,-z,relro ' --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libvo-amrwbenc --enable-version3 --enable-bzlib --disable-crystalhd --enable-gnutls --enable-ladspa --enable-libass --enable-libcdio --enable-libdc1394 --disable-indev=jack --enable-libfreetype --enable-libgsm --enable-libmp3lame --enable-openal --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libv4l2 --enable-libx264 --enable-libx265 --enable-libxvid --enable-x11grab --enable-avfilter --enable-avresample --enable-postproc --enable-pthreads --disable-static --enable-shared --enable-gpl --disable-debug --disable-stripping --shlibdir=/usr/lib64 --enable-runtime-cpudetect
  libavutil      54. 31.100 / 54. 31.100
  libavcodec     56. 60.100 / 56. 60.100
  libavformat    56. 40.101 / 56. 40.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 40.101 /  5. 40.101
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.101 /  1.  2.101
  libpostproc    53.  3.100 / 53.  3.100
Input #0, ogg, from 'The Unthanks - Magpie - Later… with Jools Holland - BBC Two-w6EIFD80f90.opus':
  Duration: 00:03:36.91, start: 0.007500, bitrate: 119 kb/s
    Stream #0:0(eng): Audio: opus, 48000 Hz, stereo, fltp
    Metadata:
      LANGUAGE        : eng
      ENCODER         : Lavf56.40.101
Failed to open file 'The Unthanks - Magpie - Later… with Jools Holland - BBC Two-w6EIFD80f90.opus' or configure filtergraph

matej@mitmanek: ~$
...
<end of log>

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Description of your issue, suggested solution and other information

My problem is that although the created file is using OGG container, it is still called *.opus. For some players (e.g., Android ones), it makes a lot of difference (and yes, just plain renaming of the file make my Android Music application picking it up). Shouldn't youtube-dl follow --merge-output-format value and use the most common file extension for it as well?

Or I may understood what the issue #5446, and that this would be a duplicate.