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/IO/ImageProvider.cs

@@ -0,0 +1,45 @@
+using System;
+using System.IO;
+using System.IO.Compression;
+using System.Linq;
+
+namespace API.IO
+{
+    public static class ImageProvider
+    {
+        /// <summary>
+        /// Generates byte array of cover image.
+        /// Given a path to a compressed file (zip, rar, cbz, cbr, etc), will ensure the first image is returned unless
+        /// a folder.extension exists in the root directory of the compressed file.
+        /// </summary>
+        /// <param name="filepath"></param>
+        /// <returns></returns>
+        public static byte[] GetCoverImage(string filepath)
+        {
+            if (!File.Exists(filepath) || !Parser.Parser.IsArchive(filepath)) return Array.Empty<byte>();
+
+            using ZipArchive archive = ZipFile.OpenRead(filepath);
+            if (archive.Entries.Count <= 0) return Array.Empty<byte>();
+
+            var folder = archive.Entries.SingleOrDefault(x => Path.GetFileNameWithoutExtension(x.Name).ToLower() == "folder");
+            var entry = archive.Entries[0];
+              
+            if (folder != null)
+            {
+                entry = folder;
+            }
+              
+            return ExtractEntryToImage(entry);
+        }
+
+        private static byte[] ExtractEntryToImage(ZipArchiveEntry entry)
+        {
+            var stream = entry.Open();
+            using var ms = new MemoryStream();
+            stream.CopyTo(ms);
+            var data = ms.ToArray();
+
+            return data;
+        }
+    }
+}

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);
+        }
     }
 }

API/Services/DirectoryService.cs

@@ -4,13 +4,13 @@ using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.IO;
-using System.IO.Compression;
 using System.Linq;
 using System.Text.RegularExpressions;
 using System.Threading;
 using System.Threading.Tasks;
 using API.Entities;
 using API.Interfaces;
+using API.IO;
 using API.Parser;
 using Microsoft.Extensions.Logging;
 
@@ -183,7 +183,7 @@ namespace API.Services
                 {
                    Name = info.Volumes,
                    Number = Int32.Parse(info.Volumes),
-                   CoverImage = getCoverImage(info.FullFilePath),
+                   CoverImage = ImageProvider.GetCoverImage(info.FullFilePath),
                    Files = new List<MangaFile>()
                    {
                       new MangaFile()
@@ -352,31 +352,8 @@ namespace API.Services
             Console.WriteLine("Processed {0} files in {1} milliseconds", fileCount, sw.ElapsedMilliseconds);
         }
 
-        /// <summary>
-        /// Generates byte array of cover image.
-        /// Looks for first valid image file (folder.png, first jpg/png file)
-        /// </summary>
-        /// <param name="filepath"></param>
-        /// <returns></returns>
-        private byte[] getCoverImage(string filepath)
-        {
-           // TODO: Sort and file type
-           // if file not zip (folder or txt)
-           using (ZipArchive archive = ZipFile.OpenRead(filepath))
-           {
+        
 
-              if (archive.Entries.Count <= 0) {return null;}
-              var stream = archive.Entries[0].Open();
-              byte[] data;
-              using (var ms = new MemoryStream())
-              {
-                 stream.CopyTo(ms);
-                 data = ms.ToArray();
-              }
-
-              return data;
-           }
-         
-        }
+        
     }
 }