Refactored GetCoverImage to a static class and unit tested it. Enhanced the method by defaulting to folder.jpg file if exists, else takes first entry.

API/Parser/Parser.cs

@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 using System.Text.RegularExpressions;
 
 namespace API.Parser
@@ -226,5 +227,12 @@ namespace API.Parser
         {
             return title.TrimStart(new[] { '0' });
         }
+        
+        public static bool IsArchive(string filePath)
+        {
+            var fileInfo = new FileInfo(filePath);
+
+            return MangaFileExtensions.Contains(fileInfo.Extension);
+        }
     }
 }