var List = project.Lists["Файлы"];
string supportedExtensions = "*.jpg,*.gif,*.png,*.bmp,*.jpe,*.jpeg,*.wmf,*.emf,*.xbm,*.ico,*.eps,*.tif,*.tiff,*.g01,*.g02,*.g03,*.g04,*.g05,*.g06,*.g07,*.g08";
foreach (string imageFile in Directory.GetFiles(project.Variables["Папка"].Value, "*.*", SearchOption.AllDirectories).Where(s => supportedExtensions.Contains(Path.GetExtension(s).ToLower())))
{
List.Add(imageFile);
}