Add brotli mime type detection to file and libmagic

[stokito]

The Linux file utility doesn't recognize brotli MIME type:

$ file -i some_archive.br
some_archive.br: application/octet-stream; charset=binary

I know, this is not related for the brotli itself but could you send a PR to file and libmagic so they start to recognize brotli mime type?

https://github.com/file/file/blob/master/src/compress.c
https://github.com/file/file/blob/master/magic/Magdir/archive

Also please confirm the correct brotli mime type #724

[eustas]

There is a little problem. Most utilities scan first file bytes and compare them against "magic" bytes.
Well,... for brotli 254 of 256 byte values are valid as the first file byte.

Brotli was designed to be "stream" format rather than file format. There is a "brotli-framing-format" project that adds tons of features (including "magic header bytes")... but it is not public yet.

[stokito]

If I understood correctly gzip is also can be named as stream format but this is not a problem for them to add few magic bytes.
I already used \xce\xb2\xcf\x81 as brotli magic bytes and application/x-brotli as mime type but asked here #724 to confirm.

Where can I read at least something about this framing format? Will we still be able to use regular .br files?

[eustas]

gzip is a wrapper for deflate streams

[stokito]

So, the magic bytes which I used are actually from another framing format from #462

[doronbehar]

I've opened an issue about this in file's bug tracker:

https://bugs.astron.com/view.php?id=111

So @eustas, the issue is that this implementation doesn't include magic bytes when it creates files and not streams?

[unphased]

What is the state of this? Most (but not all! which is not looking promising) of the brotli files i looked at start with the byte 5b...