Fixed: Don't download unknown artwork

(cherry picked from commit aecc8295c8c3ce39040d03a1f79f775e1c910362)

Fixes #2534

src/NzbDrone.Core/MediaCover/MediaCover.cs

@@ -15,7 +15,8 @@ namespace NzbDrone.Core.MediaCover
         Headshot = 5,
         Cover = 6,
         Disc = 7,
-        Logo = 8
+        Logo = 8,
+        Clearart = 9
     }
 
     public enum MediaCoverEntity

src/NzbDrone.Core/MediaCover/MediaCoverService.cs

@@ -121,6 +121,11 @@ namespace NzbDrone.Core.MediaCover
 
             foreach (var cover in author.Metadata.Value.Images)
             {
+                if (cover.CoverType == MediaCoverTypes.Unknown)
+                {
+                    continue;
+                }
+
                 var fileName = GetCoverPath(author.Id, MediaCoverEntity.Author, cover.CoverType, cover.Extension);
                 var alreadyExists = false;
 
@@ -166,8 +171,14 @@ namespace NzbDrone.Core.MediaCover
         {
             foreach (var cover in book.Editions.Value.Single(x => x.Monitored).Images.Where(e => e.CoverType == MediaCoverTypes.Cover))
             {
+                if (cover.CoverType == MediaCoverTypes.Unknown)
+                {
+                    continue;
+                }
+
                 var fileName = GetCoverPath(book.Id, MediaCoverEntity.Book, cover.CoverType, cover.Extension, null);
                 var alreadyExists = false;
+
                 try
                 {
                     var serverFileHeaders = GetServerHeaders(cover.Url);