[ARD] Mediathek Subtitle format

[a-l-e-k]

• [x] I've verified and I assure that I'm running youtube-dl 2016.04.06
• [x] At least skimmed through README
• [x] Searched the bugtracker for similar issues
• [x] Bug report (encountered problems with youtube-dl)

The subtitle in ARD Mediathek is ttml (EBU-TT-D-Basic-DE) but the downloaded file gets the extension srt. In post-processing you get this message:

[info] Writing video subtitles to: 34251192.de.srt
[ffmpeg] Converting subtitles
[ffmpeg] Subtitle file for srt is already in the requestedformat

When you try to convert the .srt named file to ass or vtt format you get this message:

[ffmpeg] Converting subtitles
ERROR: file: 34251192.de.srt: Invalid data found when processing input

[dstftw]

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