mime: add wasm architecture

This commit adds the wasm architecture to the mime package.

Updates #18892

Change-Id: I0481057bd52e39d84b3d6f5140335e293eff38f3
Reviewed-on: https://go-review.googlesource.com/106998Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/type_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux nacl netbsd openbsd solaris
+// +build darwin dragonfly freebsd js,wasm linux nacl netbsd openbsd solaris
 
 package mime