Ok, thanks for your replys. Actually I am not a coder except some very basic Java knowledge. But I managed it to create a functional Filetype plugin. I have no idea, if it is good coding style but at least it is working for me and I'd like to share this here - just in case someone is having a similar problem.
My code is based on a project template created by toe_head2001 - thanks man, that helped a lot:
using PaintDotNet;
using System.IO;
using System.Drawing;
namespace RoHoFileTypePlugin2
{
public sealed class RoHoFileTypePlugin2Factory : IFileTypeFactory
{