Files.app: Use the same file type associates as Windows on HTML files.

ui/file_manager/file_manager/manifest.json

 {
   // chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/
   "key": "MIGdMA0GCSqGSIb3DQEBAQUAA4GLADCBhwKBgQDlhpGghtnNJ7pluQN0RDwbUxwwi99oM35ZEaFYvxPLrf0fIEC18cfDdJi6u4aJ+UoSpgzK731L0P/k4LvK2Rz9kVKOy0+IvuRrWkT7lbrLfA1UEBh02OA1AAshjmyRg4IxCqgl8ia8XWq6HKegS1y1KXZYGgb4qp7Bh9VC4cIzswIBIw==",
   "manifest_version": 2,
   "name": "Files",
   "version": "3.0",
   "description": "File Manager",
   "incognito" : "split",
   "icons": {
     "16": "common/images/icon16.png",
@@ skipping 57 matching lines @@
     },
     {
       "id": "view-in-browser",
       "default_title": "__MSG_OPEN_ACTION__",
       "default_icon": "common/images/file_types/200/generic.png",
       "file_filters": [
         "filesystem:*.htm",
         "filesystem:*.html",
         "filesystem:*.mht",
         "filesystem:*.mhtml",
+        "filesystem:*.shtml",
+        "filesystem:*.xht",
+        "filesystem:*.xhtml",
         "filesystem:*.svg",
         "filesystem:*.txt"
       ]
     },
     {
       "id": "open-hosted-generic",
       "default_title": "__MSG_HOSTED__",
       "default_icon": "common/images/file_types/200generic.png",
       "file_filters": [
         "filesystem:*.gdraw",
@@ skipping 96 matching lines @@
         "background/js/progress_center.js",
         "background/js/test_util.js",
         "background/js/volume_manager.js",
         "background/js/background.js"
       ]
     },
     // chrome-extension://pmfjbimdmchhbnneeidfognadeopoehp is the image loader extension.
     "content_security_policy": "default-src 'none'; script-src 'self' chrome://resources chrome-extension://pmfjbimdmchhbnneeidfognadeopoehp; style-src 'self' 'unsafe-inline' chrome://resources; frame-src 'self' about:; img-src 'self' chrome://resources chrome://theme data: https://docs.google.com https://*.googleusercontent.com chrome://extension-icon; media-src 'self' https://*.googleusercontent.com; connect-src https://drive.google.com; object-src 'self'"
   }
 }