[devrand]

The gem is basically a database of mime type, file extension, and magic bytes. The last two are not included in the linked iana database.

[protomyth]

Where did freedesktop.org get the magic bytes? I assume (probably stupidly) that some of that has to be in a file command on some BSD.

[garblegarble]

Here's what the magic bytes look like in FreeBSD, as an example I've linked to the definition of PNG. You can see it's a fair bit more complicated but it does have mime and extension data.

https://github.com/freebsd/freebsd-src/blob/master/contrib/f...