x-ms-bmp should be flagged under "serve images under next gen formats"

[ebidel]

Discovered this while testing the `legacy-image-formats" feature policy, which warns developers about using legacy image formats like BMP.

Provide the steps to reproduce

1. Run LH on https://feature-policy-demos.appspot.com/test

What is the current behavior?

LH does not recommend that a BMP image be served in another format.

What is the expected behavior?

There's a bmp image on that page (https://homepages.cae.wisc.edu/~ece533/images/sails.bmp) that should be flagged under the "serve images under next gen formats" audit:

Environment Information

• Affected Channels: DevTools / all
• Lighthouse version: 3
• Node.js version: n/a
• Operating System: Mac os, canary 69.0.3495.0

[patrickhulce]

We target bitmaps but it looks like this one has a mime type of image/x-ms-bmp, we'll expand our regex :)

$ curl -sSL -D - https://homepages.cae.wisc.edu/\~ece533/images/sails.bmp -o /dev/null
HTTP/1.1 200 OK
Date: Wed, 18 Jul 2018 23:34:44 GMT
Server: Apache
Last-Modified: Mon, 25 Nov 2002 01:37:36 GMT
ETag: "a28b4-60436-3b03a0a357800"
Accept-Ranges: bytes
Content-Length: 394294
Content-Type: image/x-ms-bmp