Detect files that have an extension not associated with the type found by MIME sniffing

[Robbert]

We should be able to detect GIF files with an .html extension, for example.

I've asked Anne van Kesteren and it appears no JavaScript implementation of the mimesniff standard exists.

• There is mimesniff but it is in Python and was hosted on the closed down code.google.com.
• doc-sniff on npm thinks the mimesniff standard is too complicated, and thusly chose for an unsafe implementation.
• mime-sniffer doesn't implement the algorithm, it just checks a couple of byte sequences.

Depends on wesaynih/incubator#67.