Add brotli mime type detection to file and libmagic

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