Above answer from @BenGeeBee did not work for me. However I found solution under ngx-extended-pdf-viewer repo in GitHub comment.

My application is running .NET 8, so adding below to program.cs file fixed the issue.

var provider = new FileExtensionContentTypeProvider();
provider.Mappings[".ftl"] = "text/plain"; 

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