Resolve audio/x-wav to "wav" extension, rather than "x-wav" (which ffmpeg does not recognize)

[tfvlrue]

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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

This fixes ffmpeg errors when trying to downloading certain audio files from Soundcloud with the --add-metadata option set. I believe these files were uploaded without an extension, and are being identified as audio/x-wav. However, youtube-dl was downloading them with a "x-wav" extension, which ffmpeg fails to recognize as a valid output format. This change forces the extension to be "wav" which resolves the issue.

An example to reproduce the problem:

youtube-dl -o "%(title)s.%(ext)s" --add-metadata https://soundcloud.com/mediasanctuary/1033-jesse-marshall-9-12-2019

Before this change:

[soundcloud] mediasanctuary/1033-jesse-marshall-9-12-2019: Downloading info JSON
[soundcloud] 680239748: Downloading JSON metadata
[soundcloud] 680239748: Downloading webpage
[soundcloud] 680239748: Downloading JSON metadata
[soundcloud] 680239748: Downloading JSON metadata
[soundcloud] 680239748: Downloading JSON metadata
[download] Destination: 10'33' Jesse Marshall 9 - 12 - 2019.x-wav
[download] 100% of 53.24MiB in 00:03
[ffmpeg] Adding metadata to '10'33' Jesse Marshall 9 - 12 - 2019.x-wav'
ERROR: file:10'33' Jesse Marshall 9 - 12 - 2019.temp.x-wav: Invalid argument

After this change:

[soundcloud] mediasanctuary/1033-jesse-marshall-9-12-2019: Downloading info JSON
[soundcloud] 680239748: Downloading JSON metadata
[soundcloud] 680239748: Downloading webpage
[soundcloud] 680239748: Downloading JSON metadata
[soundcloud] 680239748: Downloading JSON metadata
[soundcloud] 680239748: Downloading JSON metadata
[download] Destination: 10'33' Jesse Marshall 9 - 12 - 2019.wav
[download] 100% of 53.24MiB in 00:03
[ffmpeg] Adding metadata to '10'33' Jesse Marshall 9 - 12 - 2019.wav'