/// <summary>
/// Get paths of all images (only with valid extension).
/// </summary>
/// <param name="topic">The topic</param>
/// <returns>Paths to all images of topic</returns>
private IEnumerable <string> GetImagePaths(Topic topic)
{
// get image path
string[] imageFolderPath = GetImageFolder(topic);
return(ImagePathUtil.GetValidImagePaths(FileContainer.GetFiles(imageFolderPath)));
}