Add support for .gif files (#31)

Author: detilium

src/ByteGuard.FileValidator/FileExtensions.cs

@@ -129,5 +129,10 @@ public static class FileExtensions
         /// High Efficiency Image Container.
         /// </summary>
         public const string Heic = ".heic";
+
+        /// <summary>
+        /// Graphics Interchange Format.
+        /// </summary>
+        public const string Gif = ".gif";
     }
 }

src/ByteGuard.FileValidator/FileValidator.cs

@@ -257,6 +257,14 @@ public class FileValidator
                     new byte[] { 0x66, 0x74, 0x79, 0x70, 0x68, 0x65, 0x69, 0x63 } // ftypheic
                 },
                 SignatureOffset = 4
+            },
+            new FileDefinition
+            {
+                FileType = FileExtensions.Gif,
+                ValidSignatures = new List<byte[]>
+                {
+                    new byte[] { 0x47, 0x49, 0x46, 0x38 }, // GIF8
+                }
             }
         };