use --audio-format mp3 and -o test.mp3 together cause youtube-dl fail to convert the file

[mmis1000]

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• Searched the bugtracker for similar issues including closed ones
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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 ```):

$ youtube-dl -v --prefer-ffmpeg -x --audio-format mp3 https://www.youtube.com/watch?v=AmDqeXFejlQ -o testk.mp3 
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'--prefer-ffmpeg', u'-x', u'--audio-format', u'mp3', u'https://www.youtube.com/watch?v=AmDqeXFejlQ', u'-o', u'testk.mp3']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.05.01
[debug] Python version 2.7.13 (CPython) - Linux-4.9.0-3-amd64-x86_64-with-debian-9.1
[debug] exe versions: ffmpeg 3.2.7-1, ffprobe 3.2.7-1
[debug] Proxy map: {}
[youtube] AmDqeXFejlQ: Downloading webpage
[youtube] AmDqeXFejlQ: Downloading video info webpage
[youtube] AmDqeXFejlQ: Extracting video information
[debug] Invoking downloader on u'https://r2---sn-npoeenez.googlevideo.com/videoplayback?source=youtube&clen=4870942&signature=5322CD07A076B210B5643AE4DD1095FF4F4BBD23.6762EC338A1DC601B8B5BC9793C299FDFCBDF7BD&ei=y7npWo7fO6KQz7sP3qeXmA8&key=yt6&c=WEB&ipbits=0&itag=171&ms=au%2Conr&initcwndbps=198750&mv=m&dur=290.561&mt=1525266773&id=o-APYixFEQTIxOKTR5EzXRekNA4KuRp6XYQgCgHqplGcQw&fexp=23724337&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cexpire&pl=22&mn=sn-npoeenez%2Csn-i3belne6&ip=188.166.180.85&mm=31%2C26&lmt=1449554428276624&expire=1525288492&keepalive=yes&fvip=2&gir=yes&mime=audio%2Fwebm&requiressl=yes&ratebypass=yes'
[download] testk.mp3 has already been downloaded
[download] 100% of 4.75MiB
[debug] ffmpeg command line: ffprobe -show_streams 'file:testk.mp3'
[ffmpeg] Post-process file testk.mp3 exists, skipping
...
<end of log>

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Description of your issue, suggested solution and other information

What happened?

When I use --audio-format mp3 and -o test.mp3 at same time.
the downloader will download the other format as test.mp3 and failed to convert the file because it can't override the file itself.

What did you expect?

The downloader should make a temporary file first to later convert it to mp3 instead of downloader other format directly as mp3 and get stuck.

[dstftw]

Do not hardcode extension in output template.

[mmis1000]

@dstftw
There is no reason to use extension name place holder while you only want mp3?
It also makes little sense such behavior should happen when you only want a single file

[dstftw]

Output template specifies the name of file being downloaded not an mp3 file. You can't just download mp3 cause there is no mp3 exists. You can only convert if from another format. So again: do not mess with output template if you have no idea what are you doing.