How to add thumb field to Facebook video

Question

i created desktop application that upload videos to facebook using graph.facebook and it works ok, but when i try to add "thumb" field to upload request it return error "(OAuthException - #100) (#100) Invalid image format. It should be an image file data." .
here my code used :note coverpath is full image path and i try to path image as bytes but the same error return.

dynamic parameters = new ExpandoObject();         

parameters.source = new FacebookMediaObject                             

{                                 

ContentType = string.Format("video/{0}", VideoExtension.Split('.')[1]),

FileName = VideoName                             

} .SetValue(File.ReadAllBytes(filePath));         

parameters.title = VideoTitle;         

parameters.description = VideoTitle;                    

parameters.universal_video_id = new Guid();         

parameters.thumb = coverPath;         

parameters.published = false;         

parameters.unpublished_content_type = "DRAFT";         

dynamic uploadresult = fb.Post("/367728493684998/videos", parameters);