Check file headers against a list of file signatures before
downloading the whole file and writing it to disk.
The file signature check needs some improvements (*),
but it produces usable results for the most part.
(*)
- 'webp', 'wav', and others start with 'RFFI'
- 'svg' uses the same "signature" as all XML documents
- 'webm' has the same signature as 'mkv' files
- only 'mp3' files in an ID3v2 container get recognized
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