I brokez the NetKAN. This fixes it.

Apparently the content-type is provided by the browser, so really it
could be anything for a zip file. We now check a couple more types, and
assume a `.zip` extension is also good.

I'm really sorry. This is why I should write more tests.
Merge pull request #294 from pjf/bugfix_i_broke_netkan

I brokez the NetKAN. This fixes it.