--get-filename and JSON's _filename inaccurate

[legraphista]

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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 ```):

[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'-v', u'http://www.youtube.com/watch?v=BaW_jenozKcj']
[debug] Encodings: locale cp1251, fs mbcs, out cp866, pref cp1251
[debug] youtube-dl version 2018.05.01
[debug] Python version 2.7.11 - Windows-2003Server-5.2.3790-SP2
[debug] exe versions: ffmpeg N-75573-g1d0487f, ffprobe N-75573-g1d0487f, rtmpdump 2.4
[debug] Proxy map: {}
...
<end of log>

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Inaccurate filename output:

# youtube-dl https://www.youtube.com/watch?v=vDIqXSrZ_50 -o "test.%(ext)s" --get-filename -v
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'https://www.youtube.com/watch?v=vDIqXSrZ_50', u'-o', u'test.%(ext)s', u'--get-filename', u'-v']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.05.01
[debug] Python version 2.7.14 (CPython) - Linux-4.13.0-39-generic-x86_64-with-Ubuntu-17.10-artful
[debug] exe versions: ffmpeg N-90893-gcae6f806a6, ffprobe N-90893-gcae6f806a6
[debug] Proxy map: {}
[debug] Default format spec: bestvideo+bestaudio/best
test.mp4

According to the following output, the file name should be test.mp4
But after actually downloading the file:

# youtube-dl https://www.youtube.com/watch?v=vDIqXSrZ_50 -o "test.%(ext)s" -v
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'https://www.youtube.com/watch?v=vDIqXSrZ_50', u'-o', u'test.%(ext)s', u'-v']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.05.01
[debug] Python version 2.7.14 (CPython) - Linux-4.13.0-39-generic-x86_64-with-Ubuntu-17.10-artful
[debug] exe versions: ffmpeg N-90893-gcae6f806a6, ffprobe N-90893-gcae6f806a6
[debug] Proxy map: {}
[youtube] vDIqXSrZ_50: Downloading webpage
[youtube] vDIqXSrZ_50: Downloading video info webpage
[youtube] vDIqXSrZ_50: Extracting video information
[debug] Default format spec: bestvideo+bestaudio/best
WARNING: Requested formats are incompatible for merge and will be merged into mkv.
[debug] Invoking downloader on u'https://r1---sn-4g5ednsz.googlevideo.com/videoplayback?ipbits=0&fvip=1&sparams=aitags%2Cclen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cexpire&mime=video%2Fmp4&clen=61838118&requiressl=yes&expire=1525391983&fexp=23724337&gir=yes&ms=au%2Conr&signature=8A66B07ADE7162200C1277D72F228D96990DD4A4.C0B8BF3B634471278DA9D5292BCFF64D193BE42E&mm=31%2C26&mn=sn-4g5ednsz%2Csn-5hnedn7l&ip=2a01%3A4f8%3A13b%3A1158%3A%3A2&key=yt6&source=youtube&lmt=1516257751181769&keepalive=yes&dur=193.680&mt=1525370266&mv=m&id=o-AOb04IA3R2TNAgVG7NgBJ-QCvI4xICpA9JECM9lB_mMh&ei=D07rWuTPMsjD1gLy45TQAw&itag=137&c=WEB&pl=53&aitags=133%2C134%2C135%2C136%2C137%2C160%2C242%2C243%2C244%2C247%2C248%2C278&initcwndbps=1877500&ratebypass=yes'
[download] Destination: test.f137.mp4
[download] 100% of 58.97MiB in 00:01
[debug] Invoking downloader on u'https://r1---sn-4g5ednsz.googlevideo.com/videoplayback?ipbits=0&fvip=1&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cexpire&mime=audio%2Fwebm&clen=3023455&requiressl=yes&expire=1525391983&fexp=23724337&gir=yes&ms=au%2Conr&signature=8E3A82442D4F345CCFDF4D9E883EAAE3C8C42BE8.060A713609AE5516CA15DEF669E1EBFF471AC3B8&mm=31%2C26&mn=sn-4g5ednsz%2Csn-5hnedn7l&ip=2a01%3A4f8%3A13b%3A1158%3A%3A2&key=yt6&source=youtube&lmt=1516258459881751&keepalive=yes&dur=193.701&mt=1525370266&mv=m&id=o-AOb04IA3R2TNAgVG7NgBJ-QCvI4xICpA9JECM9lB_mMh&ei=D07rWuTPMsjD1gLy45TQAw&itag=251&c=WEB&pl=53&initcwndbps=1877500&ratebypass=yes'
[download] Destination: test.f251.webm
[download] 100% of 2.88MiB in 00:00
[ffmpeg] Merging formats into "test.mkv"
[debug] ffmpeg command line: ffmpeg -y -i 'file:test.f137.mp4' -i 'file:test.f251.webm' -c copy -map '0:v:0' -map '1:a:0' 'file:test.temp.mkv'
Deleting original file test.f137.mp4 (pass -k to keep)
Deleting original file test.f251.webm (pass -k to keep)

The output file is actually an MKV

# ll
total 63372
drwxr-xr-x  2 root root     4096 May  3 19:59 ./
drwxrwxrwt 15 root root    45056 May  3 19:58 ../
-rw-r--r--  1 root root 64838367 Jan 18 07:42 test.mkv

And printing the JSON + downloading the file, it has the same issue in the _filename key

[mrfade]

https://github.com/rg3/youtube-dl#post-processing-options
--recode-video mp4

[legraphista]

Re-encoding loses time and quality.
Re-muxing is out of the question because opus is not officially supported in the MP4 container.
None of those are fixes or even related to the presented issue.
Since there is a warning thrown by youtube-dl, and the ext is mutated in the info_dict, the filename should be adjusted too:
https://github.com/rg3/youtube-dl/blob/ea1f5e5dbd6c58d4f0872a65b97611732f4b29bd/youtube_dl/YoutubeDL.py#L1877-L1880
@mrfade @dstftw

[legraphista]

It looks like the filename is updated here:
https://github.com/rg3/youtube-dl/blob/ea1f5e5dbd6c58d4f0872a65b97611732f4b29bd/youtube_dl/YoutubeDL.py#L1881-L1882
but then never updated again like it is here (in the dict):
https://github.com/rg3/youtube-dl/blob/ea1f5e5dbd6c58d4f0872a65b97611732f4b29bd/youtube_dl/YoutubeDL.py#L1708

[forresthopkinsa]

For anyone searching: this is a duplicate of #5710