Moved the test data around so more tests can use it properly. Added a IsValidArchive that is re-usable for all archive methods.

API/Interfaces/IArchiveService.cs

@@ -8,5 +8,6 @@ namespace API.Interfaces
         void ExtractArchive(string archivePath, string extractPath);
         int GetNumberOfPagesFromArchive(string archivePath);
         byte[] GetCoverImage(string filepath, bool createThumbnail = false);
+        bool IsValidArchive(string archivePath);
     }
 }