The minimum (I think) you need to do to mock the FileProvider for the Fluid templating engine

FluidFakeFileProvider.cs

namespace DeaneBarker
{
  public class FakeFileProvider : IFileProvider
  {
    // I don't think Fluid ever calls these (fingers crossed)
    public IDirectoryContents GetDirectoryContents(string subpath) { throw new NotImplementedException(); }
    public IChangeToken Watch(string filter) { throw new NotImplementedException(); }

    public IFileInfo GetFileInfo(string path)
    {
      return new FakeFile(path);
    }
  }

  public class FakeFile : IFileInfo
  {
    private string fileContents;
    public bool Exists => true;
    public long Length => fileContents.Length;
    public string PhysicalPath => null;
    public string Name => null;
    public DateTimeOffset LastModified => DateTimeOffset.Now;
    public bool IsDirectory => false;
    
    public FakeFile(string name)
    {
      // The "name" could be anything -- a URL, a database key, whatever.
      // Using that string, retrieve the file contents from... well, anywhere...
      fileContents = $"This file was supposedly at {name}.";    
    }

    public Stream CreateReadStream()
    {
      return new MemoryStream(Encoding.UTF8.GetBytes(fileContents));
    }
  }
}