mime: do not test for .wav on windows
mime: do not test for .wav on windows

Even builders don't have that mime type

R=golang-dev, dave, rsc
CC=golang-dev
https://codereview.appspot.com/7314106

pombredanne/go-deleteme,gnanderson/go,gnanderson/go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,dsriram/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang