Invalid File Extension

Below is my project structure,i am using ckfinder and ckeditor,but the problem i am facing is that i am able to upload images but when i am trying to use any files like doc,xls,pdf and click on send it to the server button,it is showing error invalid file extension.

In my code behind i have written this:

  protected override void OnLoad(EventArgs e)
        {
         
          CKFinder.FileBrowser _FileBrowser = new CKFinder.FileBrowser();
        _FileBrowser.BasePath = "/ckfinder/";
          _FileBrowser.SetupCKEditor(CKEditor1);
        }

and in Config.ascx i have changed this code

public override bool CheckAuthentication()
    {
        // WARNING : DO NOT simply return "true". By doing so, you are allowing
        // "anyone" to upload and list the files in your server. You must implement
        // some kind of session validation here. Even something very simple as...
        //
        //        return ( Session[ "IsAuthorized" ] != null && (bool)Session[ "IsAuthorized" ] == true );
        //
        // ... where Session[ "IsAuthorized" ] is set to "true" as soon as the
        // user logs on your system.

        return true;
    }

and using control like this

    <CKEditor:CKEditorControl ID="CKEditor1" runat="server"></CKEditor:CKEditorControl>

I have to use CKFinder to images and any types of file extension.Where i am doing wrong.My Solution Structure is like