Thumbnail filename extension gets ignored

[MarcSchoendorf]

• I've verified and I assure that I'm running youtube-dl 2018.03.14

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• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
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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:

[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', '--write-thumbnail', '--skip-download', '-o', '1.thumbnail', 'https://www.youtube.com/watch?v=APqFDseaF7o']
[debug] Encodings: locale cp1252, fs mbcs, out cp850, pref cp1252
[debug] youtube-dl version 2018.03.14
[debug] Python version 3.4.4 (CPython) - Windows-10-10.0.16299
[debug] exe versions: none
[debug] Proxy map: {}
[youtube] APqFDseaF7o: Downloading webpage
[youtube] APqFDseaF7o: Downloading video info webpage
[youtube] APqFDseaF7o: Extracting video information
[debug] Default format spec: best/bestvideo+bestaudio
[youtube] APqFDseaF7o: Downloading thumbnail ...
[youtube] APqFDseaF7o: Writing thumbnail to: 1.jpg

Description of your issue, suggested solution and other information

I want to decide which extensions gets used for the thumbnail. But when I put -o image.thumbnail in the command, the created file always will be image.jpg or image.png. How can I overwrite the filename extension?

[dstftw]

Short: you can't. You'll always get the actual thumbnail's extension. Forcing extensions is also completely senseless since this won't change actual image format anyway.

[MarcSchoendorf]

No this is actual quite important. I'm write a gui program which offers a thumbnail preview. When the file format can alway be different I always need to search the whole directory. With a pseudo file extension like .thumbnail I can let my ImageHandler decide which format it is. I'm sure this wouldn't be much work to do, since the %(ext)s operator is already implemented, so just deactivate it by default?

[dstftw]

Nothing stops from using pseudo file prefix though this approach itself is ugly. What you should do is parse JSON output and process thumbnails on your own.