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MIME sniffing standard defines the RAR signature as 52 61 72 20 1A 07 00.

But this signature is wrong, the RARlab spec defines the 4th byte must
be 0x21 or "!", not 0x20 or " ". Checking a rar file also indicates that:

	$ file abc.rar
	abc.rar: RAR archive data, v1d, os: Win32
	$ head -c 7 abc.rar | od -v -t x1
	0000000 52 61 72 21 1a 07 00
	0000007

There is also an issue to fix this problem in MIME standard.

See:
 - https://www.rarlab.com/technote.htm#rarsign
 - https://github.com/whatwg/mimesniff/issues/63

Fixes #30926

Change-Id: Id2e2de7ecbf7f44d37ebaf280efd05e4972c5078
Reviewed-on: https://go-review.googlesource.com/c/go/+/167781

Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>