mime: add .webp for builtin

This change modifies Go to include image/webp as a built-in mime type for the .webp file extension.

Change-Id: Id46d34fac8cc859ddd69aa8669294815654214f8
GitHub-Last-Rev: f191e1c325126e2caeb3123c912131ce5236542b
GitHub-Pull-Request: golang/go#32157
Reviewed-on: https://go-review.googlesource.com/c/go/+/178317Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/type.go

@@ -69,6 +69,7 @@ var builtinTypesLower = map[string]string{
 	".png":  "image/png",
 	".svg":  "image/svg+xml",
 	".wasm": "application/wasm",
+	".webp": "image/webp",
 	".xml":  "text/xml; charset=utf-8",
 }