Web App Manifest should to be served as application/manifest+json

[westonruter]

Per the Web App Manifest spec:

The official file extension for the manifest is .webmanifest. Some web servers recognize this extension and transfer the file using the standardized application manifest MIME type (application/manifest+json). Developers can also choose a different extension (e.g. .json) or none at all (e.g. /api/GetManifest), but are strongly encouraged to transfer the manifest using the application/manifest+json MIME type.

We're currently serving the manifest as application/json, reusing what the WordPress REST API is already using.

We need to serve it as application/manifest+json instead.

See media type registration for application/manifest+json.