Fixed: Don't download unknown artwork

(cherry picked from commit aecc8295c8c3ce39040d03a1f79f775e1c910362)

src/NzbDrone.Core/MediaCover/MediaCover.cs

@@ -10,7 +10,8 @@ namespace NzbDrone.Core.MediaCover
         Banner = 2,
         Fanart = 3,
         Screenshot = 4,
-        Headshot = 5
+        Headshot = 5,
+        Clearart = 6
     }
 
     public class MediaCover : MemberwiseEquatable<MediaCover>, IEmbeddedDocument

src/NzbDrone.Core/MediaCover/MediaCoverService.cs

@@ -147,11 +147,18 @@ namespace NzbDrone.Core.MediaCover
 
             foreach (var cover in movie.MovieMetadata.Value.Images)
             {
+                if (cover.CoverType == MediaCoverTypes.Unknown)
+                {
+                    continue;
+                }
+
                 var fileName = GetCoverPath(movie.Id, cover.CoverType);
                 var alreadyExists = false;
+
                 try
                 {
                     alreadyExists = _coverExistsSpecification.AlreadyExists(cover.Url, fileName);
+
                     if (!alreadyExists)
                     {
                         DownloadCover(movie, cover);