How Can I have IIS properly serve .webmanifest files on my web site?

For those using ASP.NET Core (I am using 2.1) you can configure the MIME types that can be served in the application Startup.cs file as per the static files docs:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    FileExtensionContentTypeProvider provider = new FileExtensionContentTypeProvider();
    provider.Mappings[".webmanifest"] = "application/manifest+json";

    app.UseStaticFiles(new StaticFileOptions()
    {
        ContentTypeProvider = provider
    });

    app.UseMvc();
}

By default, IIS does not serve any files that does not have a MIME map associated with it in its (IIS) core settings.

To address this challenge, you will need to map the .webmanifest file extension to its appropriate MIME type.

To accomplish this, open IIS and follow the steps below;

1. On the left hand side, select either your web site or the entire server in the "Connections" menu. If you select the server, your MIME mapping will apply to every web site on the server. If you select a web site, it will only apply to a single web site.

2. Next, select "MIME Types" from the IIS menu:

3. Once there, click "add..." from the right hand menu.

4. In the dialog box that opens specify .webmanifest in the file name extension box application/manifest+json in the MIME type box.

5. Click "OK".

Congratulations; you've just defined the MIME type for .webmanifest on IIS.

For Azure I added this as the web.config

<?xml version="1.0"?>

<configuration>
    <system.webServer>
        <staticContent>
            <mimeMap fileExtension=".json" mimeType="application/json" />
            <mimeMap fileExtension=".webmanifest" mimeType="application/manifest+json" />
        </staticContent>
    </system.webServer>
</configuration>