yt-dl as vivaldi-browser extension not working, possibly ffmpeg issue?

[le-mon]

• I've verified and I assure that I'm running youtube-dl 2017.05.14

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• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Question
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I wrote an extension for chrome/chromium to download youtube videos and convert it to mp3, and wanted to use it in vivaldi as well.
I set up a NativeMessagingHost in python, wich reacts to messages from the extension.
In theory as followed:

1. go to youtube
2. check if valid video url
3. send url to NativeMessagingHost
4. use embedded youtube-dl (python) to get video

chrome & chromium do what they were told to, but vivaldi doesn't like it.
Here ist the NativeMessagingHost in python:

ytconv-host.txt

the output is:
output.txt

and the logfile generated by the python script:
log.txt

I'm not sure if it's related to youtube-dl, especially the ffmpeg functions or if vivaldi has different environment var stuff etc.

I guess the problem could be traced by that line:
ERROR: Did you mean file:file:
Why is file named twice?

vivaldi version: 1.9.818.50 (Stable channel) (64-Bit)

[dstftw]

Add 'verbose': True to ydl_opts, then run ffmpeg command that fails directly from you env to figure out the reason.

[le-mon]

the script runs fine if startet directly from console.
Through nativemessaging in vivaldi something goes wrong, verbose says:

[debug][debug] Invoking downloader on 'https://r1---sn-4g5edney.googlevideo.com/videoplayback?pl=33&expire=1495477215&dur=55.681&gir=yes&ipbits=0&mime=audio%2Fmp4&itag=140&mn=sn-4g5edney&mm=31&key=yt6&requiressl=yes&clen=894573&source=youtube&keepalive=yes&mv=m&mt=1495455503&ms=au&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cupn%2Cexpire&ip=2a02%3A8070%3Ad283%3A1700%3A102f%3A2acb%3A442e%3A7f8b&lmt=1429691085092406&initcwndbps=2128750&upn=KKpWLcQuMeQ&id=o-ADn66QpLtuvyCjmVNroZwOgKmTfk2VQiu6WCmQtYat1O&ei=f9ciWe6FMsGTcKfxjIAJ&signature=2904EF8D52B9BB4E06FFBD5DF8E972314EEB3275.227159F5A815C79CCB9950E25DBF4B0AE91C5732&ratebypass=yes'
[debug][download] Destination: /home/user/Downloads/videotitle.m4a
[download] 0.1% of 873.61KiB at 495.84KiB/s ETA 00:01
[download] 0.3% of 873.61KiB at 1.24MiB/s ETA 00:00
[download] 0.8% of 873.61KiB at 2.51MiB/s ETA 00:00
[download] 1.7% of 873.61KiB at 4.68MiB/s ETA 00:00
[download] 3.5% of 873.61KiB at 2.27MiB/s ETA 00:00
[download] 7.2% of 873.61KiB at 2.23MiB/s ETA 00:00
[download] 14.5% of 873.61KiB at 3.25MiB/s ETA 00:00
[download] 29.2% of 873.61KiB at 4.79MiB/s ETA 00:00
[download] 58.5% of 873.61KiB at 5.84MiB/s ETA 00:00
[download] 100.0% of 873.61KiB at 5.98MiB/s ETA 00:00
[download] 100% of 873.61KiB in 00:00
[debug][ffmpeg] Correcting container in "/home/user/Downloads/videotitle.m4a"
[debug][debug] ffmpeg command line: ffmpeg -y -i 'file:/home/user/Downloads/videotitle.m4a' -c copy -f mp4 'file:/home/user/Downloads/videotitle.m4a'
[error]ERROR: Did you mean file:file:/home/user/Downloads/videotitle.m4a?
[error]Traceback (most recent call last):
File "/usr/lib/python3.6/site-packages/youtube_dl/YoutubeDL.py", line 1942, in post_process
files_to_delete, info = pp.run(info)
File "/usr/lib/python3.6/site-packages/youtube_dl/postprocessor/ffmpeg.py", line 530, in run
self.run_ffmpeg(filename, temp_filename, options)
File "/usr/lib/python3.6/site-packages/youtube_dl/postprocessor/ffmpeg.py", line 208, in run_ffmpeg
self.run_ffmpeg_multiple_files([path], out_path, opts)
File "/usr/lib/python3.6/site-packages/youtube_dl/postprocessor/ffmpeg.py", line 204, in run_ffmpeg_multiple_files
raise FFmpegPostProcessorError(msg)
youtube_dl.postprocessor.ffmpeg.FFmpegPostProcessorError: Did you mean file:file:/home/user/Downloads/videotitle.m4a?

I don't know if it's related to vivaldi or youtube-dl.