Since ASP.NET-Core 3.0 and above, `Microsoft.NET.Sdk.Web` MSBuild SDK doesn't automatically include `Microsoft.Extensions.FileProviders.Embedded` NuGet package anymore.

I had to add `Microsoft.Extensions.FileProviders.Embedded` explicitly.

 <Project Sdk="Microsoft.NET.Sdk.Web">
 ...
 <ItemGroup>
 <PackageReference Include="Microsoft.Extensions.FileProviders.Embedded" Version="3.0.3" />
 <!-- Or use version 3.1.2 for ASP.NET-Core 3.1 -->
 </ItemGroup>
 ...
 </Project>