Ok I found an article here which I tried now which has something to do with adding the .dll files in the Monogame Pipeline Tool.
Here is the post: [SOLVED] Cant find a default importer for tiled .tmx files - #3 by thefrostwyrm
WHen I add the Extended.Content.Pipeline.dll alone it didn’t do anything so I thought I should do the same with the Extended.Tiled but when I try to add it I get this error instantly:
System.Reflection.ReflectionTypeLoadException: Mindestens ein Typ in der Assembly kann nicht geladen werden. Rufen Sie die LoaderExceptions-Eigenschaft ab, wenn Sie weitere Informationen benötigen.
bei System.Reflection.RuntimeModule.GetTypes(RuntimeModule module)
bei System.Reflection.Assembly.GetTypes()
bei MonoGame.Tools.Pipeline.PipelineTypes.ResolveAssemblies(IEnumerable`1 assemblyPaths)
bei MonoGame.Tools.Pipeline.PipelineTypes.Load(PipelineProject project)
bei MonoGame.Tools.Pipeline.PipelineController.ResolveTypes()
bei MonoGame.Tools.Pipeline.PipelineController.OnReferencesModified()
bei MonoGame.Tools.Pipeline.CellRefs.Edit(PixelLayout control)
bei System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
bei System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
bei System.Windows.Threading.DispatcherOperation.InvokeImpl()
bei System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
bei MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
bei System.Windows.Threading.DispatcherOperation.Invoke()
bei System.Windows.Threading.Dispatcher.ProcessQueue()
bei System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
bei MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
bei MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
bei System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
bei System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
bei System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
bei MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
bei MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
bei System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
bei System.Windows.Application.RunDispatcher(Object ignore)
bei System.Windows.Application.RunInternal(Window window)
bei MonoGame.Tools.Pipeline.Program.Main(String args)
Might help somehow so I posted it as well